It is confusing, ITA.
I haven't seen an ISI synchro competition since before we moved. Are they very different in their groupings and events?
ISI groups mainly by age (with confusing names such as "junior youth, youth, senior youth, teen," [seriously, what's the difference between senior youth and teen??] etc). They also have a "formation" event. Very little actual "skating" is allowed--for example, you cannot skate backward on one foot (two-foot turns are ok, sculling backwards is ok; 3-turns, mohawks, back crossovers, back chasses are all illegal), and there are no FS elements (jumps, spins, lifts). There tends to be a lot of marching around, but it's a good way to learn the basics of skating with a team, guiding, how to keep a line straight, counting, and is great for all ages and ability levels.
ISI also has synchro skate (there are compulsories and free programs) and synchro dance. You don't have to be above or below a certain freestyle level to compete on these, which is nice.
There are also nice little categories if you don't have enough people to form a team (I think usually 8 are required) for 3-7 people: ensemble, freestyle synchro (every person on the team performs every element from a specific FS level), team compulsories (elements from a specific FS level are performed by different people on the team).