Each message line shown in Messages View contains a 'P,' 'S,' 's,' or 'R' code. These codes have the following meanings:
The message was posted to the queue with the PostMessage function. No information is available concerning the ultimate disposition of the message.
The message was sent with the SendMessage function. This means that the sender doesn't regain control until the receiver processes and returns the message. The receiver can, therefore, pass a return value back to the sender.
The message was sent, but security prevents access to the return value.
Each 'S' line has a corresponding 'R' (return) line that lists the message return value. Sometimes message calls are nested, which means that one message handler sends another message.