This counts how many inbound frames on this circuit arrived already flagged Discard-Eligible by the customer's own equipment, meaning the CPE itself decided this traffic was low-priority and safe to drop first under congestion. A rising cktInDEFrames count relative to cktInFrames tells the admin what fraction of a customer's traffic is being self-throttled at the source, useful context when explaining occasional drops to that customer. For example, if 30% of inbound frames on a circuit are DE-marked, the admin knows that circuit's CPE is deliberately marking that portion as expendable rather than the network selectively dropping it.