Normative language
In documents or sections that explicitly adopt this convention, uppercase normative terms carry the meanings defined in RFC 2119 and RFC 8174. They turn prose from description into requirement.
The definitions below follow RFC 2119 and RFC 8174. Uppercase normative terms have normative meaning only in documents or sections that explicitly adopt this convention.
MUST
An absolute requirement of the specification.
MUST NOT
An absolute prohibition of the specification.
SHOULD
A recommended behavior. Valid reasons may exist to ignore it in particular circumstances, but the implications must be understood and weighed.
SHOULD NOT
A recommendation against a behavior. Valid reasons may exist to do it in particular circumstances, but the implications must be understood and weighed.
MAY
A permitted option. A document may choose whether to include the item.
When to use normative language
Use uppercase normative terms only where a statement is genuinely normative. FTL does not use normative language in exploratory brainstorming merely to make prose look precise, and it does not allow a rewriting tool to invent MUST where the source only said can or could.
FTL-W11 preserves source modality. FTL-W07 keeps requirements out of descriptive prose. Both rules depend on the definitions above.