The traditional way for SIP trunking service is to provide X trunks and to apply a monthly fee per trunk (so the total charge is X * monthly fee), and the limit of simultaneous calls that the customer can have = X. However, this approach is not very customer-friendly - the customer may have a need for more simultaneous calls than X during some special periods (Christmas season, discounted sale, etc). Of course the customer could buy 2 * X SIP trunks to have those periods covered, but they would have to pay monthly for some trunks that they don't use most of the time.
So a more customer-oriented approach was invented - the customer gets 2*X SIP trunks from day 1, but they pay monthly only for those trunks that they actually use (maximum concurrent calls within a month). So one month it can be X, another - X+3, and another X-2.