This appears to refer to the services to be supplied by the property manager. Although tenant change is of course involved (unless the apartment is new), the action that in this case will be performed by an outside agency is letting or reletting.
On the contrary, the manager will almost always be involved in the tenant change once the apartment has been let.
The definition in the Woerterbuch Immobilienwirtschaft is "re-letting; granting a new tenancy; letting new premises"