TIdSync = class(TObject);

TIdSync is a TObject descendant that implements support for abstract Synchronized methods calls that have parameters and/or return values. TIdSync also provides methods that support thread-based notification messages to reduce overhead in server applications. 

TIdSync is used to act in behalf of the Indy thread instance specified in Thread, and also ensures that at least a single instance of the Thread class has been created in the application. 

An instance of TIdSync is used when TIdThread calls Notify, and is added to the global thread-based notifcation mechanism in GNotifyThread.