IMO: As long as you protect the integrity of the sterile item you may place an item on top of a securely wrapped package. It is not the best practice but can be done. You would not place something that would have the possibility of leaking onto or puncturing the sterile pack. You would not "store" non-sterile items "on top" of sterile supplies. Hope that answers your question.
The information you are looking for is published by the Association for the Advancement of Medical Instrumentation (AAMI). The AAMI produces recommended practices for sterilization and storage and are the accepted authority on how to process, sterilize and store supplies. You can probably find the book(s) in processing or whoever manages it at your facility should have a copy.
It is extremely POOR practice to place a non-sterile item on top of (or underneath) a sterile item. In fact, really, they should even be on a different shelf, or at least next to each other and not touching. Sterile items should be stored in a clinically controlled environment and should not be stored in the same areas a non-sterile items.
You really want to get technical? Sterile "have available" items should not be in the OR with the patient, but should be right outside the room in the restricted area.
Why? The pressure from items on top of each other are considered an "incident" and can cause a pressure exchange of air from outside of the sterile package to inside of the sterile package.