What does "expressions of opinion" mean?
Expressions of opinion should be supported by reference to the authority on which they are made and the verifiable facts which support them.
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An expression is the process of transmitting ideas, such as thoughts, concepts, or feelings, between entities (such as individuals or groups).
An opinion is a judgement or view; thus it is not a fact or truth.
An "Expression of Opinion" is used in your example to distinguish between an "Expression of Fact", which would not necessarily need the references that are required for an "Expression of Opinion".