The card schemes named in companies' disclosed card programs — Visa, Mastercard, Amex, and regional networks like UnionPay, JCB, and RuPay. Unlike every other leaderboard here, a network isn't a vendor a fintech chooses among competing options — it's the fixed rail a card-issuing program rides on, which is why it's counted separately from the vendor-tally universe (see lib/tally.ts) even though the ranking is built the same way.