function SelectRead(
    const ATimeout: Integer = IdTimeoutInfinite
): Boolean; override;

Boolean - True when a socket handle is available for reading.

SelectRead is an overridden Boolean function that implements the method used to access the Select() API for a protocol stack implementation. 

SelectRead is a convenience method used to indicate if there are readable socket handles for the protocol stack. SelectRead returns True when the list of socket handles ready for reading would contain a positive non-zero value. 

ATimeout indicates the number of milliseconds to wait for successful completion of the Select() API call. 

Use Select or SelectReadList get the list of socket handles.