Falloff
In the image above you can see how the various Falloff types work. If you select Spline you can use the spline to define a custom falloff (see also Funktionsgraphen).
This setting can be used to affect the Falloff types Bell and Needle with regard to how soft/har and slow/fast the falloff effect should occur.
If Falloff is set to Spline, a custom falloff effect can be defined. The function graph ranges from X=0 as the tool’s center to X=1, the tool’s radius.