Most fees have their own field — commission, yard, insurance, the checkoffs, vet, manure. Custom fees are for anything else your barn charges that doesn't fit those, such as a tagging fee or a special-handling charge. Each one carries onto the consignor's settlement alongside the standard deductions.
Reach for custom fees only when no standard field fits — using the built-in fields keeps your settlements clearer.
Technical section
Data model, API, and internals — visible to developers.