Hello Thom,
this is Agus, also from AudioStellar.
So, the software doesn't have a play/stop button. For that reason, the 2 ways to stop it, would be to clear the trajectory or mute the Unit.
In the case of the "Sequence Unit", there is a slight difference. Sequence units follow the global tempo. This tempo can't be stopped. But, if you sync AudioStellar with other software/hardware by using the Midi Clock option in the Settings menu, then the global tempo will follow the other software/hardware, and in that case, it is possible to Play/Stop the Sequence Units from this other software/hardware. For example, if you Midi sync with Ableton Live, every time you play or stop in Ableton, Sequence Units will play/stop. The same will be if you use an external sequencer, like an Arturia Beatstep, for example.
Although it is not possible yet, we are planning to make "Trajectories" follow the global tempo too.
Hope this helps. Any other suggestions, feel free to write.
Agus