Relationship ​between NRW and UF Water

I do research ​in the field of ​water policy so ​I would like to precisely ​understand ​the relationship ​between non-​revenue ​water (NRW)​ and unaccounted-​for water (UFW)?. What is the ​difference ​between UFW and ​NRW?  ​ Could you ​please give ​some illustrative examples?