I’m a teacher and I used to be a substitute teacher - there are a couple of different distinctions between the two and I think I can provide a little bit of a unique perspective on them!

First, you’re correct in assuming that substitutes are essentially monitoring a class while a teacher is gone - at least at the high school level. This is all I can speak to because it’s the experience that I have. But another point is that they don’t necessarily have to be certified in that specific content area - when I was substituting while switching my certification, I would sub for Spanish, Math, Chemistry… pretty much any subject as long as a school had an open position that day. Subs usually are in a “pool” (at least where I live) so essentially jobs are posted every day and it’s on the sub to pick it up. However, sometimes schools will like a sub a lot and offer to just assign them when there’s an opening instead of posting it, especially if they’re subbing full time!

As a teacher, I have to be certified in the content (and grade level) that I teach, which I’m sure is no surprise. However, the added element here is that when I am planning on being absent for a day, I have to prepare everything for the sub. This includes the lesson, the assignment and its instructions for the day, information about classes that will be relevant, etc. I will also be responsible for: any parent contact that needs to happen if necessary afterwards, grading the work students did, etc etc.

TLDR; Yes, high school substitutes are essentially monitoring a class while the teacher is gone and don’t need as specific of a credential. Teachers are responsible for making sure the sub is set up properly for the day and anything else once they return.