I have had experience with a join table in an inventory database for sales inventory. A LINES table is the JOIN table for PRODUCT sales. In this set up any time you have a sale there is a line. However I have not come across this yet and want to see if my thinking is correct.
What happens when a LOCATION:: has no TIERS:: but still houses an ITEM:: could you just have a tier called SELF or NONE? I am going to play with it and see what I come up with. I wanted to see how other would handle this. Could you have a table occurrence for LOCATIONS:: with TEIRS:: and another for LOCATIONS:: with ITEMS::. Thanks