It is "Start Playback" in the Assign Actions category of Remote Control, in Settings.
I have taught ST3 to begin playback (Start Playback), via a midi sent signal, which is logged in ST3 (in the Assign Actions) as "MIDI START".
With this, i can begin a song on my RC600, by simply hitting any footswitch on the RC600, and this begins the playback on the corresponding ST3 song (mostly lyrics to scroll). This is brilliant, and makes it incredibly easy to coordinate the 2 units.
As mentioned earlier, the only issue is that if the song on ST3 has finished (e.g. lyrics have scrolled to the end and stopped), and i am still playing the same song on the RC600, and i do a Stop/Start on the RC600 (which often happens for effect, having a short break of 1 or 2 beats in the song etc), then this triggers the ST3 to jump to the next song. The next song on ST3 does not start playing yet, however the midi instructions in that next song, (e.g. [midi: PC49@2]) are sent to the RC600, which then sets it onto the next song also.
Ideally what I require is for the mentioned Assign Action of "Start Playback", to only do that, Start playback, and do nothing else. So if the song on ST3 is at the end, then nothing happens, as it is at the end.
I hope this makes sense, and thank you for taking the time to reply and attempt to understand this issue, I do appreciate it greatly.