Depends on what you want to do with it.
Nagura perform two basic functions: contributing mud, or eroding another stone. The former can be divided into abrasive-dominant or buffer/lubricant mud, and the latter might be divided into surface conditioning and slurry raising.
If you're using a soft white nagura, you'll probably want to use it on a harder base that doesn't contribute much mud and might run a bit glassy.