Read the rules. "Scale" isn't divided up that way, and "semi scale" isn't a classification.
In scale, you're mostly trying to duplicate the full-size prototype as exactly as possible, and you get a higher static score the better you do.
In Designer Scale (which is the closest to your "scale" that one can come) the judges can get right up to your plane for static judging.
In Sport Scale the judges can't get closer than 15 feet. Sport scale is intended to be a more relaxed event, obviously.
A semi-scale stunter isn't any official class -- it's just a design approach to stunt competition where a competitor, either because he thinks he'll garner more points, or because of personal preference, chooses to make a model that more or less resembles some full-size prototype. There's no rule to keep someone from building a Nobler and claiming that it's a semi-scale Cessna 120 -- there's just no advantage, either.
You could, in theory, build a semi-scale stunter and enter it into both stunt and scale competition. But the two classes have mutually exclusive requirements (the aerodynamic requirements of stunt don't match the outlines of many full size prototypes), not to mention the fact that scale ships tend to be heavier to get more static points. So you probably wouldn't do well at either, assuming that you could get into the scale competition at all.
The semi-scale stunt plane designer, in order to avoid building a semi-stunt scale plane, must basically build a "stunter in full scale clothing" -- i.e., you've got to choose a prototype plane that sorta-kinda has a general arrangement that is close, then you have to modify the outline into this massive compromise that flies well, but still has people looking at it and seeing the bird that you intend for them to see. It's nothing like building a scale airplane, where ideally the outline of the full scale airplane dictates the outline of the model.