This command calls the operating sytem command RegisterWindowMessage() to obtain the identifier of the name you specify. It then sends a message with that identifier and the two other arguments.
The message is sent to the window that was targeted with TargetWin, SendWin, etc.
If the current send mode is SendMessage, the message is sent with SendMessage(). Otherwise it's sent with PostMessage().
The numeric arguments can be written in decimal or hex. An example of hex notation is shown below.