That’s the hay loft. Probably more so on older barns, but that’s where the term is from.

You probably don’t need to add holes to the shelves. There are pegs that go into the adjustable holes that are flattened on one side, and the shelf just rests on them. Should be able to get a set of universal ones from the hardware store for pretty cheap if the originals are missing.

You’re probably gonna have to experiment a bit. For baking and more delicate things probably the listed fan temp, but for things like roasted potatoes it doesn’t matter as much.