It all depends on how the entries are distributed.
For instance, if there are enough juniors to make a viable division, they will have a separate Junior division, but if there are not quite enough juniors for a full division, they will add the youngest of the Adults, until they have enough. It will be CALLED “Open”, but will really be “juniors and young adults”.
I doubt that the secretary looks up individual records, but they usually have a pretty good idea of who are pros, who are experienced/successful, and who are inexperienced/ less successful, and try to group them together. So you may get one “Open” division that is mostly pros and experienced amateurs, and another “Open” division that is mostly less experienced riders (or horses).
Technically, it supposed to be split randomly, but it is pretty clear that often doesn’t happen.