In my company, we often need to write requirements for our customers. They are pretty much recommendations (i.e., we do not impose anything), but given a certain baseline, some are stricter than others: they can be mandatory, desirable or optional. To avoid misinterpretation, we use the terms defined by RFC 2119. This question focuses on the first definition:
- MUST This word, or the terms "REQUIRED" or "SHALL", mean that the definition is an absolute requirement of the specification.
So, in principle, either of the three words could be used exchangeably. I tend to use must, whereas some colleagues tend to favor shall. For example, I'd write
Every car MUST have a key.
Every car SHALL have a key.
However, the latter (shall) resonates odd, unnatural and even pretentious to my ears. From other questions in the site and my previous experience, I get different ideas:
- It sounds refined, but old fashioned.
- As a question, it is just a proposition (shall we go later to...?).
- It is seldom used, but in laws and other official documents.
So my question is: is there something that speaks in favor or against using one or another option (i.e. MUST or SHALL)? And slightly related: am I being too critical of this usage of SHALL in modern English?