I always thought it was for degrees per frame, but that doesn't work out right as far as the game goes. Looking back at it, perhaps the d stands for some fractional degrees instead? So 25 DPF could be for 0.025 or 0.25 degrees per frame of video.

Michael C. Fortner
"Maxim 37: There is no such thing as "overkill".
There is only "open fire" and "I need to reload".