A count of Controlled Slip Seconds — seconds in which a controlled frame slip (a timing re-synchronization event) occurred — recorded under the vendor-defined ('User') supplemental counter set, accumulated over the full lifetime of the interface, i.e. since the system was last powered on or the counter was last reset. Slips usually point to a clocking mismatch somewhere in the network, so an admin watching this counter climb would check the interface's transmit-clock configuration and the timing source upstream.