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The differences are only in emphasis; nominally both must and shall express a mandatory rule, as the definition in RFC2119 says.

Traditionally, shall implied the power to define reality. An emperor would not simply utter a wish or an order; he would describe the future reality, because he had absolute power. For example, the Mexican emperor's ruling stated "the sentence shall be executed within twenty-four hours". The only answer was "consider it done".

In modern times such a decree is issued by a judge or, as here, by a technical regulation. Neither can, to the same degree, define reality; but they have uncontested power to define regulations.

Therefore, in modern use the emphasis of shall is that it creates a requirement for compliance with a regulation. For example, in the C++ standard draft we find the sentence "a conforming implementation shall [...] accept and correctly execute that program. The C++ committee issues a decree, very much like an emperor: It defines a future reality, in its limited domain. Each conforming implementation shall (as in a future action) do that because if it didn't, it would not be conformant. It's a little less clear in legalese like "both parties shall try to settle the dispute": This is the behavior defined by decree in this contract. If they don't, they are in violation (not conformant, so to speak).

Interestingly, because the modern institutions, including ISO standardization committies, are democratic by nature, this modern use of shall is a bit more of an agreement than a simple must. A contract has been signed by both parties. A C++ implementation author decided that it should be conformant, etc.

Must, by contrast, is neutral: It can be mandatory for any number of reasons, including forces of nature. It is not necessarily a decree.

The differences are only in emphasis; nominally both must and shall express a mandatory rule, as the definition in RFC2119 says.

Traditionally, shall implied the power to define reality. An emperor would not simply utter a wish or an order; he would describe the future reality, because he had absolute power. For example, the Mexican emperor's ruling stated "the sentence shall be executed within twenty-four hours". The only answer was "consider it done".

In modern times such a decree is issued by a judge or, as here, by a technical regulation. Neither can, to the same degree, define reality; but they have uncontested power to define regulations.

Therefore, in modern use the emphasis of shall is that it creates a requirement for compliance with a regulation. For example, in the C++ standard draft we find the sentence "a conforming implementation shall [...] accept and correctly execute that program." The C++ committee issues a decree, very much like an emperor: It defines a future reality, in its limited domain. Each conforming implementation shall (as in a future action) do that because if it didn't, it would not be conformant. It's a little less clear in legalese like "both parties shall try to settle the dispute": This is the behavior defined by decree in this contract. If they don't, they are in violation (not conformant, so to speak).

Interestingly, because the modern institutions, including ISO standardization committies, are democratic by nature, this modern use of shall is a bit more of an agreement than a simple must. A contract has been signed by both parties. A C++ implementation author decided that it should be conformant, etc.

Must, by contrast, is neutral: It can be mandatory for any number of reasons, including forces of nature. It is not necessarily a decree.

The differences are only in emphasis; nominally both must and shall express a mandatory rule, as the definition in RFC2119 says.

Traditionally, shall implied the power to define reality. An emperor would not simply utter a wish or an order; he would describe the future reality, because he had absolute power. For example, the Mexican emperor's ruling stated "the sentence shall be executed within twenty-four hours". The only answer was "consider it done".

In modern times such a decree is issued by a judge or, as here, by a technical regulation. Neither can, to the same degree, define reality; but they have uncontested power to define regulations.

Therefore, in modern use the emphasis of shall is that it creates a requirement for compliance with a regulation. For example, in the C++ standard draft we find the sentence "a conforming implementation shall [...] accept and correctly execute that program. The C++ committee issues a decree, very much like an emperor: It defines a future reality, in its limited domain. Each conforming implementation shall (as in a future action) do that because if it didn't, it would not be conformant. It's a little less clear in legalese like "both parties shall try to settle the dispute": This is the behavior defined by decree in this contract. If they don't, they are in violation (not conformant, so to speak).

Interestingly, because the modern institutions are democratic by nature, this modern use of shall is a bit more of an agreement than a simple must. A contract has been signed by both parties. A C++ implementation author decided that it should be conformant, etc.

Must, by contrast, is neutral: It can be mandatory for any number of reasons, including forces of nature. It is not necessarily a decree.

The differences are only in emphasis; nominally both must and shall express a mandatory rule, as the definition in RFC2119 says.

Traditionally, shall implied the power to define reality. An emperor would not simply utter a wish or an order; he would describe the future reality, because he had absolute power. For example, the Mexican emperor's ruling stated "the sentence shall be executed within twenty-four hours". The only answer was "consider it done".

In modern times such a decree is issued by a judge or, as here, by a technical regulation. Neither can, to the same degree, define reality; but they have uncontested power to define regulations.

Therefore, in modern use the emphasis of shall is that it creates a requirement for compliance with a regulation. For example, in the C++ standard draft we find the sentence "a conforming implementation shall [...] accept and correctly execute that program. The C++ committee issues a decree, very much like an emperor: It defines a future reality, in its limited domain. Each conforming implementation shall (as in a future action) do that because if it didn't, it would not be conformant. It's a little less clear in legalese like "both parties shall try to settle the dispute": This is the behavior defined by decree in this contract. If they don't, they are in violation (not conformant, so to speak).

Interestingly, because the modern institutions, including ISO standardization committies, are democratic by nature, this modern use of shall is a bit more of an agreement than a simple must. A contract has been signed by both parties. A C++ implementation author decided that it should be conformant, etc.

Must, by contrast, is neutral: It can be mandatory for any number of reasons, including forces of nature. It is not necessarily a decree.

The differences are only in emphasis; nominally both must and shall express a mandatory rule, as the definition in RFC2119 says.

The emphasis of shall is that it creates a requirement for compliance with a regulation. For example, in the C++ standard draft we find the sentence "a conforming implementation shall [...] accept and correctly execute that program. Each conforming implementation shall (as in a future action) do that because if it didn't, it would not be conformant. It's a little less clear in legalese like "both parties shall try to settle the dispute": This is the behavior defined by decree in this contract. If they don't, they are in violation (not conformant, so to speak).

Shall therefore also is a bit more of an agreement than a simple must. A contract has been signed by both parties. A C++ implementation author decided that it should be conformant, etc.

Must is neutral: It can be mandatory for any number of reasons, including forces of nature, and it can be mutually agreed upon or not.

The differences are only in emphasis; nominally both must and shall express a mandatory rule, as the definition in RFC2119 says.

Traditionally, shall implied the power to define reality. An emperor would not simply utter a wish or an order; he would describe the future reality, because he had absolute power. For example, the Mexican emperor's ruling stated "the sentence shall be executed within twenty-four hours". The only answer was "consider it done".

In modern times such a decree is issued by a judge or, as here, by a technical regulation. Neither can, to the same degree, define reality; but they have uncontested power to define regulations.

Therefore, in modern use the emphasis of shall is that it creates a requirement for compliance with a regulation. For example, in the C++ standard draft we find the sentence "a conforming implementation shall [...] accept and correctly execute that program. The C++ committee issues a decree, very much like an emperor: It defines a future reality, in its limited domain. Each conforming implementation shall (as in a future action) do that because if it didn't, it would not be conformant. It's a little less clear in legalese like "both parties shall try to settle the dispute": This is the behavior defined by decree in this contract. If they don't, they are in violation (not conformant, so to speak).

Interestingly, because the modern institutions are democratic by nature, this modern use of shall is a bit more of an agreement than a simple must. A contract has been signed by both parties. A C++ implementation author decided that it should be conformant, etc.

Must, by contrast, is neutral: It can be mandatory for any number of reasons, including forces of nature. It is not necessarily a decree.