'nat_ref' can be used if there is *the* reference number of an object (like the ref of a street). For more special numbers, ref:XY should be used. 

Using ref:RO:LMI might be a good idea, but is not required. 'RO' should be in capital letters as it is a country code.

Taginfo says that there are already a couple ref:lmi, but no ref:ran.

https://taginfo.openstreetmap.org/keys/ref%253Almi

So, I would use the same 'ref:lmi' here, and for ref:ran / ref:RO:ran / ref:RAN / ref:ran no one decided yet what to use.
