You can use XGen guides to place, shape, and orient primitives generated on polygon meshes. Think of guides as proxy versions of primitives, which determine where primitive generate and how they appear. You can specify that you want to use guides when creating a new description. You cannot use guides when generating groomable splines. See Create hair or fur with spline primitives.
Use the Add or Move Guides tool 
to place guides on faces of polygon surfaces. You can place any number of guides on each face and any area of a polygon mesh, including curved and sloping areas. By default, guides display as orange flat curves in the scene view. See Shape spline primitives using XGen guides
In the Outliner, each guide is represented by this icon 
and a unique ID.
After adding guides to a mesh surface, typical workflows include shaping the guides by manipulating their CVs. When you preview the Description, the generated primitives are influenced by the guide shape.
With guides you can:
- Manipulate the CVs on each guide. See Shape spline primitives using XGen guides.
- Add or delete guides. See Work with XGen guides.
- Move guides anywhere on the polygon surface. See Work with XGen guides guides.
- Copy a guide's shape to other guides. See Work with XGen guides.
There are a number of tools in the Utilities tab that you can use to shape and manipulate guides.