Primitives attributes define the properties and appearance of the geometry that is being generated on a patch. Use the Primitive Attribute sections to edit Spline, Sphere, Card, and Archive primitives.
For attributes with numeric values, you can use Ptex maps, expressions, or slider controls to set its value. By default, attributes appear in the XGen window with slider controls. Click the 
icon and select Create Map ( see Create a Ptex map to control primitives),Load Expressions, or Reset to a slider to set attribute values using these available methods.
Click the 
icon to open the XGen Expression Editor, which lets you create new expressions or load one from the expression library (see Control primitives using sliders and expressions and XGen Expression Editor).