Price Difference: If the Purchase Order lists the product at $5.50 and the Vendor Bill comes in and is recorded for $5.60 then the $0.10 difference will be booked into this account.

Counter-Part Account: If you change the cost of a product you have in inventory from $1.50 to $1.25, then the $0.25 x quantity in stock will be booked to this account (because you need to record via the Inventory Loss Counter Part account the "other side" of the Journal Entry - one side is "value of inventory goes down" and the other side is "value of inventory lost goes up")

You would normally use two different accounts.

One represents "differences between what my Vendor tells me during the PO negotiation compared to the Bill they send" and one represents "gains or losses when I revalue inventory".