Use UV sets to define where the hair goes and to control its distribution and density.
By default Hair uses UV set map1, but you can link your hair systems to different UV sets on your polygonal surface. For example, if you have different UV sets defined for different projections, you can link the hair systems to the UV set that gives the best result.
In the Relationship Editor, when you link a hair system to a UV set, all the hair systems attached to the polygon are linked to that UV set. However the UV set is controlled by the Map Set Name attribute in the Extra Attributes section in the follicleShape, so you could define it from there. (Currently you cannot edit the follicle’s Map Set Name attribute in the Attribute Spreadsheet.)
Currently you need to create a UV set for hair that is between zero and 1. You can use regions outside this range for areas you want to be bald. Or if you simply do not define UVs for a triangle, then it will be bald. You should do this before creating the hairs, as on creation of hair, hairs are rejected that are located at undefined UV locations. If the UV for a follicle becomes undefined after creation, then the follicle will simply move to the center of the object; however, you could then manually select and delete these follicles. See Create your own hair on surfaces.
To link attached hair systems to a different UV set
The left panel lists all the hair systems that are attached to the selected polygon.
The right panel lists mesh nodes with UV Sets for the selected polygon. If more than one polygon is selected, only the last selected polygon is listed.