Commercial banking runs on negotiation. A corporate client gets a bespoke fee schedule, a volume commitment, a rate concession, or all three. That agreement is real, it is signed, and it is almost never something the core billing system can express.
So it gets handled around the edges. Terms live in a signed PDF and a spreadsheet. Someone applies manual adjustments each cycle. When that person moves teams, the institutional memory of why a particular client pays what they pay goes with them.
The costs land in three places. Clients are billed incorrectly and notice, which is a relationship problem. Concessions granted years ago never get revisited, which is a revenue problem. And when an auditor asks why this client is charged differently, assembling the answer means finding the right email, which is a governance problem.