This read-only counter reports frames whose transmission was deferred (delayed due to a busy medium) on the internal CPU party-bus port -- the "party bus port" being the internal, Ethernet-like management link between the switch ASIC and the CPU. It is exactly the kind of standard RMON/Ethernet-style statistic (size-bucket, error, collision, or pause counter) an admin would check on any physical interface, but scoped here to the internal CPU uplink, so a rise in the error/discard variants points at a problem in CPU-to-ASIC communication rather than on any customer-facing port.