PR Value in the top command

Hi All, i was trying to understand the the meaning of the PR field in the "top" command. PR field is the kernel notion of importance of a task. It is loosely coupled with the "nice" value, which is a user-space concept. Roughly they are related with.. PR=20+nice (-20<= nice <=19) which means, PR can never go below 0. As an experiment to understand this even more, i created an user-app with real-time scheduling policy (SCHED_FIFO) and ran this application. while the application was running i triggerred "top" and what i see that the PR value for my application displays a -ve value. what does this mean. If at all this is correct, what are the valid range of PR which i should expect?