Rules and recommendations do not consistently use the words MUST, SHOULD, SHALL. They SHOULD.

The commonly agreed rules are the ISO ones described here, repeated below:

The following definitions apply in understanding how to implement an ISO International Standard and other normative ISO deliverables (TS, PAS, IWA).

"shall" indicates a requirement
"should" indicates a recommendation
"may" is used to indicate that something is permitted
"can" is used to indicate that something is possible, for example, that an organization or individual is able to do something

In the ISO/IEC Directives, Part 2, 2021, 3.3.3, a requirement is defined as an "expression, in the content of a document, that conveys objectively verifiable criteria to be fulfilled and from which no deviation is permitted if conformance with the document is to be claimed."

In the ISO/IEC Directives, Part 2, 2021, 3.3.4, a recommendation is defined as an "expression, in the content of a document, that conveys a suggested possible choice or course of action deemed to be particularly suitable without necessarily mentioning or excluding others."

Furthermore, CAN should seldom be used for Rules. Hence a phrasing using CAN should be a Recommendation rather than a Rule (see for example DENV_FORMAT).