Escrow
An on-ledger contract that locks XRP until a date, used by Ripple to cap monthly sales.
Escrow is a protocol feature: XRP is locked to a condition or a release time and the ledger enforces it.
In December 2017 Ripple locked 55 billion XRP into a queue of monthly escrows releasing up to 1 billion each. Unsold XRP is re-escrowed, so the schedule is public and auditable rather than a promise.