top rack = generally used for?
middle rack = used mostly for ?
bottom rack = good for ?
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Ovens vary quite a lot in their specific performance characteristics, but in general, the heat source is at the bottom, and things on low shelves tend to brown more on the bottom. Heat builds up near the top, and things on high shelves will tend to brown more on top, and be a little safer from burning on the bottom- important for things like cookies with chocolate in them. But mostly it's a matter of experience with your own oven and the things you bake.
I agree with smaug- you have to know your oven. My conventional oven has a serious hot spot on the lower rack towards the back left side. Even convection ovens will have spots that are a little cooler or hotter than the temperature says. It tends to be a matter of personal preference for something like a baked ziti. I like mine a little crunchy on top and the cheese browned up- so I go top rack (a bit cooler in mine) with it so I can give it a couple extra minutes under the broiler without overcooking it. I like my cookies very crisp, so I usually go lower rack with them.