License usage is usually based on currently active sessions - based on the receipt of a Radius Accounting Start message. For example, with plus, a client that matches an Authz policy that has a profiling condition will consume 1 license.
You can usually click on the license usage bar and it should open up a new window with the current active sessions report for that license. You can also export this report if you want to.
Sessions are removed when the Radius Accounting Stop is received. I know that there is a session removal period after which all the old inactive sessions are removed from ISE, which then clears up the license usage. I believe it is 5 days but I am not sure on that.
