If I have a FAM with nothing more than links to HUSB, WIFE, and a few CHIL, and I select Edit Family (not the raw), I get a popup with two edit boxes. The first is GUID, and the second DATE. If I enter a date, it becomes a subrecord of the GUID.
If I put the raw GEDCOM back to original, then add only one _NMR and save, then select edit family again, the pop-up has the same two at the bottom, which behave the same.
Before those, it has fields which create, in order, DATE, DATE, PLAC, ADDR, AGNC, TYPE, AGE. All of these create level two subrecords of the _NMR tag.