Judged precision flying similar to pattern (F3A- NSRCA (more acronyms!- Natiobal Society of Radio Controlled Aerobatics). My understanding is that IMAC grew out of F3A or pattern to open up competition for Scale aerobatics. F3A is very specific on size and type of plane ( more RC based designs) whereas IMAC is truly "Minuature" Aerobatics competition. from a distance you might think you were at Fond du Lac watching an actual Acro competition. Categories include Basic, Sportsman, Intermediate, Advanced and Unlimited (there are"known" routines- published now and "unknown" that pilots see a day before the competition. Some competitions offer a Freestyle performance (will include 3D and more personalized performance, similar to an air show routine). Beyond Basic (any plane can be used) the scale must be within 10% on all surfaces and design. You will see Extras, Yaks, Edges, Pitts, Caps, etc. anything you would see at an international aerobatics competition that can prove the 10% scale. Size is often 30cc or larger beyond Basic. There are two great sources- mini-iac.com and a great new .99 cent iPhone and Android app- IMAC lite. At App Store.
Wether you want to compete or just get more precise with your flying the IMAC resources are wonderful tools. Try them out! To date I think international membership is just about 1000. So it is still a relatively small community. There are competitions in every region of the country and worldwide as well.
That's probably more than you wanted to know but I just got back into the hobby and recently "found" IMAC and I'm hooked! Gotta practice on the sim now! good luck.