I believe it stands for National PRomotional rates. You may also want to try for LPR which are local promotional rates.
Look at the rate rules before you book your magical $0 rate. There may be some special stipulations that you may not be able to meet. On the opposite end, I have seen some $9,999 rates using those promotional codes.
If you want to use a standard reward, why not just book it as such and be done?