This applies to the dark part of the GND filter / the ND filter as well
1 stop filter - allows 50% light transmission
2 stop filter will only allow 25% of light transmission
3 stop filter will only allow 12.5% of light transmission
and so on and so forth
Hi MRSAMO
I think you might be referring to ND filters for the " smooth " effect that you talked about.
For GND filters :
( Picture from bobatkins.com )
The darker half is like an ND filter, and is placed across the scene to reduce the bright areas of the scene that will otherwise overwhelm the ( dynamic range ) digital sensor. Very useful and I always carry them around.
Ryan