That is to be able to create the shelves, indicate the floor on which it is, etc.
You can create use Location to represent shelves. But if you plan to have a lot of locations, you should prefer as much as possible flat layout and activate the “Flat Children” option. This will allow Tryton to use optimized queries to compute stock quantities (which otherwise is some thing like O(d * l) where d is the depth of the tree and l the number of element per level).