In my experience 'B' grade can mean:- open box, a customer returned item, a RMA item that has either been repaired or replaced by the manufacturer, an item that has been used for test purposes, surplus stock, products with missing parts.
Do not assume that the item will work, I have had a few 'B' grade items that were DOA.
I purchased a 'B' motherboard a few years ago and someone had had ago at repairing the DIMM sockets on the back of the board, several of the pins had solder shorts on them. If I had put my CPU and memory in it and powered it up it would have gone up in a puff of smoke taking my CPU and memory with it.