OnSystat is a TIdUDPSystatEvent property that represents the event handler signalled to build the active user list for client connections on the server. 

OnSystat is triggering during execution of the context for the client session in OnUDPRead. Applications must assign a procedure to respond to the event notification, or the user list returned to the client is always empty. 

Arguments passed to the OnSystat event handler procedure include the socket handle used to read from and write to the client session generating the event, and the TStringList used to build the active user list for clients connected to the server. The active user list is written to the client connection in OnUDPRead.

TIdSocketHandle OnUDPRead