I have some info on this. All it measures is hard hitstun, pummels, and attacks that happen extraordinarly close to each other (so rapid jabs). While Mario's downthrow into utilt is guaranteed at least once, it doesn't register this for two reasons: the first is that the game doesn't count "light" hitstun as part of a combo except it seems in certain cases (like jabs). The second reason is that it doesn't count the very very short period of vulnerability after hitstun ends before you are able to effectively escape (so the very first few frames of an airdodge/spot dodge/roll or the frames before an aerial's hitbox comes out).
So generally speaking there are a couple of frames after the combo starter stops counting in which you could still land an attack (though this depends on who you are hitting because some characters have extraordinarly fast nairs like Ness and Yoshi)
EDIT: I should also say that the combo counter doesn't count grab confirming as a combo. So while if you hit someone with the first hit of Falcon's nair and then grab them, it is impossible to escape if spaced properly, the grab resets the combo counter. This counts in every grab confirm situation (Falcon's bair, Palutena's jab, etc)