The Variable option is available when you click Apply to create a chamfer. If you have selected the path, but not created the chamfer, select the Advanced button to create a variable distance chamfer. If you select this option, you can edit the distance along the last chamfer created.
The Law option uses a combination of the distance methods to define nine options. The first word specifies how the distance varies as it enters a line; the second word specifies how the distance varies as it leaves a line: