Multiple unit auctions
This describes auctions in which there are two or more (usually identical) items for sale per lot. In some multiple unit auctions bidders can bid only for one of the items, while in others bidders can bid for whatever quantity they desire.
If all items for sale in a multi-unit auction are exactly identical then it is described as a “homogenous” auction. If the items are different in any way then the auction is “heterogeneous”.
Single unit auctions
This describes auctions where only one item is offered for sale per lot.
How to tell the difference
If more than one item is for sale in a lot but participants can bid only for the whole collection, then this would be a single unit auction. However, if participants can bid individually on the separate items that make up the collection then this would be a multiple unit auction.