This message doesn't contain any data. The system sends it as a reply to a message that the receiving thread's chain of BHandler does not recognize. See MessageReceived() and ResolveSpecifier() in the BHandler class of the Application Kit.

This message doesn't contain any data. It's sent as a default reply to another message when the original message is about to be deleted. The default reply is sent only if a synchronous reply is expected and none has been sent. See the SendReply() function in the BMessage class of the Application Kit.

This constant identifies a message as being a reply to a previous message. The data in the reply depends on the circumstances and, particularly, on the original message. For replies to scripting messages, it generally has a result field with requested data and an error field with an error code reporting the success or failure of the scripted request.