Can, Could, May, Might, Must, Should

Can means the ability to do something. Avoid could.
May means permission to do something. We rarely indicate permission to do something in the documentation. Use can for the ability to do something and might for the possibility of something.
Might means it’s possible to do something.
Must means it’s necessary to do something. Avoid should. Use must for necessity or rephrase as “Salesforce recommends...” when a customer ought to do something.

Salesforce Style Guide for Documentation and User Interface Text