Our signature card states in the Acknowledgement section, "I am a US citizen or other US person."

Is there a reg that clarifies the defintion of "other US person?"

It states it on your signature card in what context? If you have a question about content, you should be asking whoever designed your signature card.

I found my answer.

http://www.irs.gov/Individuals/Internati...S.-Tax-Purposes

So you were talking about a W-9 and not a signature card.

Resident Alien is an "other US Person".

W-9 substitute embedded as part of the signature "card."