In the latest discussions about curves/hair mesh attachement information (T95776), it was decided to use UV coordinates to store where on the mesh each root is. For that, we have to specify which of the UV map attributes to use for UV lookups. This property isn't used yet, but it will be shortly when refactoring the attachement information in the add brush and the to particle system conversion. Differential Revision: https://developer.blender.org/D15115