A client reaches nothing until you add an application to it and pick a level.
Levels are a ladder, and each rung includes the ones below it. Mail runs Off, Read, Draft, Manage, Full. Calendar runs Off, Read, Full. Anything else is on or off.
Changing them
Open the client, then Applications. Changes apply to every connection on that client at once, from its next call. Nothing needs reconnecting.
There is no Save. Each change is saved as you make it, and the next call an assistant makes uses the new level.
When an assistant asks for one
An assistant that needs a permission it does not have will say so, name the application and the level, and give you this page. It cannot grant itself anything, and a link it hands you never carries a change in it: opening a link only ever shows you the page. You make the change.
More than one account
If you have connected several accounts of the same kind, each is its own application with its own permissions, and calls say which account answered. An assistant that could mean more than one will ask which, rather than guess.