ReadList is a read-only TIdSocketList property that contains socket handles with used for the Connection and OutboundClient connections for the proxied client session. 

ReadList is updated and used during Server processing that triggers the OnConnect and OnOutboundCOnnect event handlers for the client context. 

ReadList is also use to populate the list of socket handles in DataAvailList.

NetData DataAvailList Connection OutBoundClient TIdSocketHandle TIdMappedPortTCP.OnConnect TIdMappedPortTCP.OnOutboundConnect