ref:nid=* is a key indicating reference number of the object in the national registry of heritage in Poland.
Description
This tag value should be number of heritage available in NID data.
Heritage registries administrated by voivodeships in Poland is collected by NID (National Heritage Institute) for convenience they dataset is popular to use in Poland[1].
Unfortunately those numbers are not unique, one should not rely on them as unique identifier.
Mapping
This tag should be added to object already tagged with heritage=*.
Polish language version of heritage=* page, provides detailed info about tagging heritage objects in Poland.