At a high-level, we categorize messaging types by the nature of user interaction:
System messaging alerts the user of important system-related issues or status. It’s initiated by the system and it’s not a result of the user’s actions. e.g. System maintenance coming up this weekend.
Engagement messaging nudges the user to enter or update data in the system. It can be initiated by either the system or another user. e.g. No activity on Opportunity x in the past 30 days. Create a task or schedule an event.
Access messaging appears when the user tries to access an item that’s not available to them. It may be because the record has been deleted, the user doesn’t have access, etc. etc. e.g. Lead x isn’t available.
Feedback messaging is the application’s response when the user is interacting with it. The majority of create, read, update, delete (CRUD) actions will result in feedback messaging. e.g. Account x was created.