From Valve Developer Community
- Distance at which the prop starts to fade (<0 = use fademaxdist).
- Max fade distance at which the prop is visible (0 = don't fade out).
- If you specify a fade in the worldspawn, or if the engine is running under dx7, then the engine will forcibly fade out props even if fademindist/fademaxdist isn't specified. This scale factor gives you some control over the fade. Using 0 here turns off the forcible fades. Numbers smaller than 1 cause the prop to fade out at further distances, and greater than 1 cause it to fade out at closer distances.