A purge/scavenge-task works in the background. It checks the sessions and compares them to what is indicated to be reserved from ISL. If a seat is found to be reserved from ISL, but has no related valid session in the database, the software will surrender the seat to ISL. If the surrender fails, it is tried next time of clean-up.
Do not delete rows from license seat tables manually or update them in some way, because the clean-up job may not return the seats to the license machine and they remain reserved from ISL "forever".