B-stock can truly mean anything unfortunately.
Floor model, extra inventory, old items, returns, refurbished, etc. It is basically a superset that can include all types of inventory with any number of explanations. A-stock is generally considered brand new, in unopened factory packaging. Anything less than that, no matter what it is, could be called b-stock.
It is important to know why an item is called b-stock, but quite often there is a full warranty with b-stock items that covers you if there are issues with the product when you get it.
Other terms:
NOS - New Old Stock - A prior year model that is still in the unopened factory box.
NIB - New In Box - Anything that is in an unopened factory box