Hi,

You can do this using a selection tag.

1. Add a selection tag containing a polygon selection

2. Add hair to the whole object

3. Link the selection tag to the Input Mesh parameter in Guides from Mesh and the Selection Tag parameter in Hair from Guides

4. Enable Use Face Include in Hair from Guides.

Step 3 will ensure that the guides are only generated on the polygon selection, and that Hair from Guides only considers the selection for hair root distribution. 

Test scene attached. 

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Attached Files:

HairToSelection.c4d (207132 bytes),

Jeordanis Figuereo (Product Designer. EPHERE Inc.)