procedure Add(
    AHandle: TIdStackSocketHandle
); override;

Add is an overridden procedure in TIdSocketListLinux that implements the abstract method from the ancestor class. 

Add calls the Linux API function FD_SET to ensure that the socket handle in AHandle is included in the current read, write, and exception descriptors. 

Use Remove to delete a socket handle from the socket handle container. 

Use Items for read access to existing socket handles in the list.