Per its DESCRIPTION, this Counter64 counts the seconds this port was in an available (in-service, not failed) state. Because this is a lifetime counter that only resets on an explicit clear, a reading here means the circuit has stayed usable for that many seconds on this OC-n (SONET) port, accumulated over the port's entire service life, not merely since the last reboot. For example, an Available Seconds count that keeps pace almost exactly with Total Time, aside from a handful of Unavailable Seconds, is what a carrier points to when proving a 99.999%-availability SLA.