Is the split-op choice only for reserve?

I'm 17 and am enlisting within due time (going through BCT this summer and then going through AIT after senior year) for (hopefully) MP, and I've heard I can ONLY do reserve or something as the such? I'm confused because I KNOW I must do some active duty (I think a min of 4yrs) eventually. I think when people are saying that they are refeering to the year that ill be in school well be reserve, obviously? I'd just like some illuminating on this subject please.