Destination is a string property that represents the endpoint or identity of the connection for the IOHandler. Destination is used to represent the concatenation of values from the Host and Port properties in the form: 

"127.0.0.1:001" 

In IOHandler (a base class), Destination is a simple string property. In descendant classes (like TIdIOHandlerSocket), Destination is used to read from or write to the Host and POrt properties for the IOHandler.

TIdIOHandlerSocket.Destination