That "STOP" is pretty self-evident. It's where the rail starts to curve. Even without the stop line, it'd be hard to go any farther. If it were me and without a stop line I'd have to stop at same place on bottom clamp it won't go past that curve.
Originally Posted by
Sy Reene
I understand the question as to whether in his instance the top or bottom of his clamp can go to the "Stop" line on the saddle rail. Eg. pic below. I'd definitely at least my case assume it's the bottom clamp surface as shown.