This system gives you the full stock from the supplier that supplies the store. You can see anything and get it ordered, no problem. Certain items may have issues that cause it to take longer to bring in, but at least you can get it - right?
Preorders are unreleased items. There is no way for anyone to have any of these items because they aren't yet released. You can get the jump on them by preordering them. When the products come in, you will get them ASAP.
These are items that are not normally stocked at the distribution center. Any of these items will be added to the next order from the supplier in question. It usually takes a bit longer, but it beats not being able to order them at all!
These items are not stocked at the distribution center. They could be on the way from the supplier, so there shouldn't be any worries.