Comedogenic just means an ingredient tends to clog pores. It might be too rich, too occlusive, or physically block the follicle, and any of those can lead to a breakout.
Labels aren't held to a standard
This matters more than most people realize, because labels lie to you, or at least they're not held to any real standard. “Non-comedogenic” and “oil-free” are not regulated terms. A product can say either one and still contain ingredients that clog acne-prone skin.
So instead of trusting the front of the bottle, you actually have to look at the ingredient list, which is exactly the kind of thing this app is built to help you do.
Where pore-cloggers hide
Comedogenic ingredients also hide in places people don't think to check. Haircare is a big one. Conditioners, styling creams, oils, and pomades can transfer onto the forehead, temples, jawline, neck, or upper back and cause breakouts in exactly those spots. The same goes for lip balm, toothpaste, even facial tissues.
If you're breaking out along your hairline or around your mouth and can't figure out why, it's worth looking at what's touching your skin that isn't technically “skincare.”