Every stock transaction in the system belongs to a Location. Stock Management is by Location.
When we set up an Item we don't usually allocate it to a Location straight away as you may be unsure which location will have that Stock Item. The first time we have transactional data for an Item we will specify a Location and this will automatically update the StockLocation table. Each stock item can reside in any number of Locations.
Bins
Bins are not preset but are freeform at the time of Data entry. They are attached to a Location so the Location must already exist. The first time you use a Bin it adds to a list which will be part of the dropdown for that Location in the future. This can be updated at time of Item maintenance or at time of transactional entry.