What does "Bidding Is Per Unit" mean?

Sometimes an auction lot will consist of multiple identical items - for example, this listing is for 40 sheets of siding: 

On this lot, the affiliate has set up the bidding on a per unit basis. This means that the bid amount is for one sheet of siding, and the bid is automatically multiplied by the quantity indicated (also known as placing a "times the money" bid).

The total amount of the current high bid can be seen on the item detail page, but the "next required bid" is still the per unit amount.