Written into the contract. Identical for every client at the same level. Never adjusted privately.
An advance against future services. Not repaid in cash — it stays with you and with your family.
Every clause has to be readable aloud to the client without a second of discomfort. If it fails that test, it does not go in.
Fixed terms give up the margin that hides inside a quote. It only works if you are paid for readiness rather than volume.