"settings" isn't documented at https://bridges.torproject.org/info#settings
On relay-search, we have some bridges which have been assigned the distribution pool "settings" by rdsys. For example, here is one of the lines from the bridge-pool-assignments/ file on collector:
29e1f1ffbc4f4b306f3e3ffb52f58757620ea34c settings transport=obfs4 ip=4 blocklist=ru
This causes relay-search to list "settings" as the distribution method (which is ok, since that's what the pool file said), and relay-search links users to https://bridges.torproject.org/info#settings for an explanation of what it is. But that page doesn't have any explanation.