If no traffic is seen from the client based on the seconds in the inactivity timer, the user will be removed from the user table. Enabling the "deauth inactive users" parameter means the access points will also send a deauth to those clients.
Clients are not removed immediately from the user table; the inactivity timer determines when users are removed. Enabling the "deauth inactive users parameter" will also send a deauth to those clients, as well.