Milestone Billing — Bills against project progress — a deposit at kickoff, an installment at delivery, the balance at acceptance — rather than a calendar. Milestone completion signals from fulfillment or PSA trigger each invoice, and rev-rec tracks the same events.
Milestone Billing lives in the Rate & Bill phase of Fulfill & Bill — the stage where you activate what was sold, meter what's used, bill it right. In the corpus tool index this phase maps to the Billing and Paymentscategories.