This confused me at first too.
You're correct that the "one step" action is for an internal action in OpenPnP. It's not a "perform the next placement and then pause" button, like in the Charmhigh software.
OpenPnP's behavior is *much* more useful when debugging problems with a machine because it allows you to single-step through the process, and check logs etc to see what has happened.
Ideally it would be nice if there were 2 one-step buttons:
* Debugging single-step, like the existing button.
* Operational single-step, which runs through the sequence to the end of the next placement and then pauses.
I suspect it's not a very important feature though.