From Valve Developer Community
Start Fade Dist/Pixels
- Distance at which the prop starts to fade (<0 = use fademaxdist). If 'Screen Space Fade' is selected, this represents the number of pixels wide covered by the prop when it starts to fade.
- End Fade Dist/Pixels
- Maximum distance at which the prop is visible (0 = don't fade out). If 'Screen Space Fade' is selected, this represents the *minimum* number of pixels wide covered by the prop when it fades.
- Fade Scale
- 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.