Head count is simply how many animals make up the lot. It drives per-head pricing and per-head fees, and it is the number the auctioneer calls when selling "so much per head."
If a lot is split or a buyer takes only part of it, the count that sells can differ from the lot's head count. The sale transaction records its own head count for exactly that reason.
Technical section
Data model, API, and internals — visible to developers.