As for the difference between one time events and subscription events, this is the same reason that IFTTT introduced a “do button .”
SmartThings allows you to subscribe to events like a switch turning on or a motion sensor being triggered.
The original rule machine, as I understand it, was based on responding to events.
Then people wanted the ability to trigger a one time execution of a rule just like turning on a switch. Or like the Do button. No subscription, just a manual initiation.
That led to a second app which was called, I think, “triggers.”
And that in turn led to requests for one time events that would have conditionals around them.
The purpose of the one time event is that they could then be used from other parts of SmartThings. It was no longer all automatically driven by subscription events.
And then the conditionals let you say “but only if” other conditions were true.
In my experience, many UIs do distinguish between subscription events and “at will” events, whatever you want to call them. But other UIs will just count an at will request as a different type of subscription. I don’t think it really matters one way or the other, it’s just a question of what fits your particular design.
IFTTT added the do button not so much because of the “do” part as because of the button. It’s essentially the same as a recipe without an if other than the at will request. And they knew they wanted that at will request to have a button UI. So they went ahead and set it up separately.