Ripple has locked away its treasury supply of XRP in 55 separate escrow contracts that expire each month for the next 55 months. Every time one of these contracts is released by the XRP Ledger, the associated XRP is made available to Ripple. Ripple decides how to utilize this released XRP in its business each month: It then places each unused portion of the released XRP back into a new escrow for another 55 months.
References:
https://ledger.exposed/escrow-stats
https://ripple.com/insights/ripple-to-place-55-billion-xrp-in-escrow-to-ensure-certainty-into-total-xrp-supply/