This reports the current membership status of this specific link within its parent port channel -- for example whether it is actively forwarding traffic as part of the bundle or excluded from it. This is the single field that tells an admin, member by member, which physical links are actually contributing bandwidth to the channel right now versus which ones are technically present but not participating, which directly explains an under-performing but 'up' channel. A membership value of 'inactive' on one of four members explains a channel that looks 'operState=up' overall but is only actually carrying traffic on three links.