XGen patches define the areas of the geometry that generates primitives. Patches bind to the faces of the selected polygon mesh when you create the Description. Patches provide information about the geometry, such as face ID's, face size, position, and face normals, which the Description then uses to place and control the primitives. Collection (.xgen) files store the information about the patches (see XGen files).
In the Outliner, patches are identified by this icon: 
. In the scene view, areas bound to XGen patches display with a magenta-colored X at the center the geometry faces. Patches
do not have attributes to modify. You can change which polygon faces bind to the patch by using the Bind Face menu options.
When you create a Description, Maya adds xgenPref attributes (Extra Attributes section) to the polygonShape node of the mesh bound to the patch. These attributes store initial state information about the patch and geometry. This information ensures that the density of the primitive generation remains consistent when the geometry deforms.
By default, primitive generation is based on the number primitives per unit area (which for XGen is cm) on the mesh. If geometry deformations change face sizes, the number of primitive generated on the mesh also change. These changes in primitive generation cause unwanted popping between frames as the geometry deforms.
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