Used for User32.SetWinEventHook See here
public enum WinEventHookFlags : int
The DLL that contains the callback function is mapped into the address space of the process that generates the event. With this flag, the system sends event notifications to the callback function as they occur. The hook function must be in a DLL when this flag is specified. This flag has no effect when both the calling process and the generating process are not 32-bit or 64-bit processes, or when the generating process is a console application. For more information, see In-Context Hook Functions.
The callback function is not mapped into the address space of the process that generates the event. Because the hook function is called across process boundaries, the system must queue events. Although this method is asynchronous, events are guaranteed to be in sequential order. For more information, see Out-of-Context Hook Functions.
Prevents this instance of the hook from receiving the events that are generated by the thread that is registering this hook.
Prevents this instance of the hook from receiving the events that are generated by threads in this process. This flag does not prevent threads from generating events.