Can, May, Should, Must

Can (past tense could) means the ability to do something.
May (past tense might) means permission to do it, or suggests the possibility that something might happen.
Should means the reader ought to do it—simply tell the reader to do it, and eliminate the should.
Must means necessity or probability.
Use may only when giving permission; otherwise, use can, must, or might.

Salesforce.com Style Guide for Documentation and User Interface Text