P The message was posted to the queue with the PostMessage function. No information is available concerning the ultimate disposition of the message.
S 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.
s The message was sent, but security prevents access to the return value.
R 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.