[Koha] RLIN

I can't find anything in the documentation about RLIN, nor can I find it in the sql schema, so am looking for some assistance, please. First (enquiring mind) what does the abbreviation stand for? Secondly, if I understand correctly, it is used for indexing authority entries, but how can it be searched? Example: the default 730 for "serials/continuing resources" shows only a single input line "9 (RLIN)" - obviously, I can add $a to get a searchable "UNIF_TITLE", but there must be some reason for the default to hide this option. Thanks - Paul