This read-only counter tallies frames received with an unknown protocol, and the description notes this is only ever incremented for connections using service interworking in translation mode, staying at zero for every other connection type. Because translation mode has to actually inspect and translate the encapsulated protocol, unlike transparent or network interworking which just pass bytes through, a nonzero count means frames are arriving carrying a network-layer protocol this switch's translation table doesn't recognize, so those frames get discarded rather than forwarded. A customer running an uncommon or legacy layer-3 protocol over a translation-mode interworking connection would see this counter climb while their more common IP traffic on the same connection passes through fine.