Sex class narrows a lot below species — for cattle it might be steer, heifer, cow, or bull; for sheep lamb, ewe, ram, or wether; and so on by species. Like species, it must be uniform across the lot. A mixed pen gets the MIXED class.
Buyers bid on uniform groups. Keeping a lot to one sex class is what lets the auctioneer call a single weight and price for the whole lot.
Technical section
Data model, API, and internals — visible to developers.