It's the division between disputed transactions and processed transactions, usually within the month. Each card network publishes its own limit and its own monitoring program for those who go past it.

Two details change the math and are almost always overlooked. The first is the lag: the dispute arrives weeks after the sale, so a month of rapid escalation drags down the following month's index. The second is that a voluntary refund doesn't enter the numerator, and neither does a pre-chargeback resolved in time.