Rolling reserve
A slice of every payout that the acquirer holds back for a set period as collateral against future chargebacks.
The acquirer pays merchants within days, but customers can dispute purchases for months. If the merchant goes bust in between, the acquirer is left covering the refunds. The fix is to keep, say, ten percent of each payout for 90 or 180 days. The money is then released on a rolling basis, one day's withheld amount at a time, hence the name.
Reserves fall hardest on travel firms, event organisers, subscription services and new merchants with no history: anyone who sells something long before delivering it. For a young online shop they can swallow the entire working capital. Payment facilitators such as Stripe and PayPal have often imposed them without warning, which has spawned an entire genre of furious forum threads.