When is a sailboat the stand on vessel in relations to a recreational power boat?

The stand-on vessel is the one that is on the starboard side of a boat, and its red light or streamer is visible to you. The situation where a sailboat would be the stand-on vessel will be then the sailboat is overtaking the power boat.

The opposite of a stand-on vessel is the give-way vessel, which is on the port side, the green light or streamer of which is visible to you.