Yes, the use of the word "must" is an accepted term in your documentation. In the English language, both "shall" and "must" are verbal expressions that indicate that something is mandatory. ISO International Standards, like ISO 17025, used for conformity assessment contain requirements and use "shall" to describe a mandatory requirement. For example, the laboratory shall document the competence requirements for each function.
ISO standards only use the word "must" to refer to an obligation on the user of the document (e.g ISO 17025) due to a country -unique condition or law, or law of nature;, not a requirement of the ISO standard. For example, all buildings in the active seismic area of Los Angeles must be earthquake-resistant.
This is by agreement of definitions in terms of an ISO directive.
In the English language "must" is typically used in everyday speech as a command, necessity or request. That is the reason why "must" is used in a procedure to instruct the user on what is necessary to comply with, so the laboratory can meet a mandatory ISO 17025 "shall "requirement. For example, The laboratory manager must retain the records for determining the competence requirements.
IF you wish, you can define the use of the word “must” in your quality manual or procedures as a mandatory instruction to fulfil the ISO 17025 “shall” requirements.
Note too, that anywhere in your documents, you state something “is” or somethings “are” then this is also an mandatory expectation that needs to be met in terms of assessment during internal auditing and for accreditation, i.e. the evidence “must” be available.
For more information on ISO 17025, see the White paper Clause-by-clause explanation of ISO 17025:2017 at https://info.advisera.com/17025academy/free-download/clause-by-clause-explanation-of-iso-17025