Received 90-Day Notice
The Proper Way for a person to move out without any repercussions is by giving a 90-day notice and moving on the date written on the 90-day notice. (Date must coincide with the lease end date.)
As soon as the key is returned then the apartment is officially considered a vacant apartment, as stated on the lease contract.
If a 90-Notice is not turned in then and the tenant moves out, it is one of the other scenarios (Skip, Eviction, Transfers).