That means something sent a WM_COPYDATA message to StrokesPlus. I only added that message box for someone (or anyone else) who was needing to test sending WM_COPYDATA messages to other programs.
Something on your system is sending the message to S+, not sure what or why, but that message only appears in response to S+ receiving the message.
However, I'll look into making that an option so it will only pop up the message if the option is turned on since there's no good reason for that to always be on for everyone.