I want to know

Are you trolling?

No

I really want to know 

What's nm?

Oh I see but why only usa What's their rating standard

Clearly the OP does not get out much. LOL

It is a valid question to start a discussion. As a FM, i reckon OP knows the difference.

NM´s are a thing of the past. The title became obsolete with the introduction of the international titles `FIDE Master´and ´Candidate Master´. 

Thanks guys

Various national federations award the NM title. I think there is a Ukrainian NM title that is generally stronger than FIDE FM (or close to it) but the US NM, at the lowest end, is closer to FIDE CM in strength. 

In many places in the US, it is harder to get a FIDE rating, let alone a title.

 Thanks alot,martin for the info