interchange
Interchange is the fee paid on each card payment by the merchant's acquirer to the bank that issued the card. The schemes set it; the acquirer passes it on. In the UK, consumer debit is capped at 0.2% of the transaction and consumer credit at 0.3%.
Interchange is the largest fixed component of what a business pays to take cards, and the one nobody in the chain can negotiate: the schemes set the rates, the issuing bank receives them, and the acquirer passes them through. The UK Interchange Fee Regulation caps consumer debit at 0.2% and consumer credit at 0.3% where the shop, the acquirer and the card issuer are all in the UK. Commercial cards and cards issued abroad are not capped, which is why a card-not-present payment from a European customer can cost several times a domestic one.
For a software platform this matters because interchange sets the floor under any fee the platform earns. A pricing calculator that assumes 1.4% on everything will overstate margin on commercial cards and understate it on domestic debit. Ask for the mix of card types across the platform's customers before estimating what moving it is worth.
For a business checking its own bill, the question is whether the statement shows interchange separately. Under blended pricing it is folded into one rate, and the PSR found most small merchants could not see what they were paying for.