You can use expressions to generate values for modifier attributes, masks and regions maps. For example, you can create an expression that randomizes the cut amount of a Cut modifier or the amount of curl a Clump modifier applies to hairs.
Attributes that are controllable by expressions have these icons , in the Attribute Editor as they are also controllable by texture files and painted textures.
For example, for a Cut modifier connect it to the Amount attribute.
See XGen Expression Editor for more information about loading and editing expressions.