I was wondering about this too. I could not find any clear documentation on this. I had a look in the code and if I interpret it right then any internal user that has logged in in the last 15 days is counted towards the license. Interestingly, the code in v10 seems to filter on the active flag as well while the code in v9 does not. So this could mean that in v9, if you deactivate a user and create a new one, both users will be counted towards your license for a short period of time.
An internal user is any user that is part of the "Employees" group.
Logic in v11 seems to be the same as in v10.