Its not a tag. Its a XREF (cross-reference). All “level 0” records have them, so sources, media objects, families, repositories, shared notes as well as individuals have them. They cross reference the record to other records that are linked to them.
Also known, in the GEDCOM spec as xref_ID and a “pointer” (it “points to” or links to other records). Most info is on pages 16-19.