Getting back the the original question, the answer is it depends on the contest. In Scalemasters there is expert, team and open class. Expert class requires you to be the "builder of the model". What does that mean you ask? Well basically you have to have done most of the work. You can legaly start with an arc or even an arf, and strip the covering and refinish it and be good to go, from there it's all the way through kits, plans built and scratch built. So while you can legaly compete with a plane that started life as an arf, you won't be competitive regardless of how good a pilot you are. The ARC/ARF's work fine for Open class though, where all you need is a minumum of documentation for a maximum of 30 points static score. The same builder rules apply to team as expert, just you have a team mate do the flying. AMA is similar except instead of open class they have a fun scale class, which is open to any scale model regardless of petigree.