SOT isstnds for “Start of time” - https://github.com/synfig/synfig/blob/master/synfig-core/src/synfig/time.cpp#L68-L85
This is a reference to first frame on the timeline.
There is also
EOT - “End of time” (last frame of timeline).
As I can see, for some technical reasons, if some vertex has animated on/off states, then Synfig forces to auto-insert a “virtual” waypoint at
In our case the
0f waypoint duplicates
SOT waypoint. I guess this is a technical issue.