I am assuming you are talking about a page like this
http://www.symantec.com/security_response/writeup.jsp?docid=2009-050707-0639-99? Where it says Risk Level 2: Low.
Well I am not sure there is any documentation on the specifics of what gives a virus a rating of low vs very low ect. At least not any publicly available ones. I know that it is based on quite a few categories such as:
1. Rate of spread
2. Intent of the virus ie) functionality of the virus
3. Percent infected
4. Ease of removal
I know there have to be others but I think you get the general idea. Where you looking for more specific information? I can look into it a bit more if you would like me too : )
Thanks
Grant