Please have a look at https://wiki.osm.org/wiki/Key:ref. The keys in the group ref are supplemented by a country (or possibly organisation) abbreviation for better validation and to avoid different meanings. This is followed by a detailing according to the person responsible for the data, in your case as suggested ref:US:eff. The prefix unsigned only indicates that the value cannot be found on the mast (if a mapper wants to check it). In this combination they are of course new, but the structure allows an interpretation of the meaning even without detailed knowledge.