The lot number is what the auctioneer announces and what the ring clerk records the sale against. It is an integer assigned per lot. Two lots in the same sale can never share a number, but the same number can reappear in a different sale.
Lot number is not the same as ring order. The number identifies the lot; the ring order decides when it sells. They often differ — lots can be renumbered or combined without changing the sequence they run in.
Technical section
Data model, API, and internals — visible to developers.