This tool lets you draw a line on skin geometry, which stops any generated hair from being influenced by guide curves on the other side of the line. This is useful for getting a clean split along a line, for example where guides have already been groomed in opposite directions.

Clicking Part Guides on the Guide Process tab will put down a Draw Curve SOP and Guide Partition SOP where you can paint on the skin geometry. For more information, see the Guide Partition SOP help page.