We call "must" and "must not" words of obligation. "Must" imposes an obligation on the readers to tell them something is mandatory. "Must not" are used to say something is prohibited.
While "shall" and "shall not" are not the most suitable terms for obligation, if you see the terms used in these policies, they have the same meaning as "must" and "must not".
"May" means purely optional and does not imply that the writer recommends that option to the reader. "Should" also means optional but implies that the writer recommends and advises the reader to use that option.
Must = mandatory
Must not = prohibited
May = optional
Should = recommended
These are the only valid word choices to convey those meanings.