IOHandler is a TIdServerIOHandler property that represents the input / output handler used by listener threads that accept client connections for the server. IOHandler is used to get the TIdServerIOHandlerSocket instance assigned to the property when the server becomes Active. 

Applications can assign another TIdServerIOHandler descendant to the property prior to setting Active to True. Assigning a value to IOHandler cause an existing IOHandler instance to be freed prior to updating the property value. The IOHandler is also updated to reflect the current Scheduler for the server. 

TIdListenerThread uses the IOHandler for the server when the listener thread is executed. The IOHandler is used to accept the client connection and to create a TIdIOHandler instance used in the TIdTCPConnection that represents the client connection. 

IOHandler is freed in Destroy when an instance has been assigned and ImplicitIOHandler contains True.

TIdServerIOHandler 

TIdServerIOHandlerSocket 

TIdListenerThread 

Active