Custom attributes that are stored by the compound for internal purposes. You can also use them later in your tree or in another tree, for example, you can get StrandToMesh_NbSections if you want to know the number of sections in the extrusion.
In particular, StrandToMesh_ExtrusionID is an integer per vertex that stores the index of the extrusion. All vertices that were extruded along the same strand have the same index. This lets you distinguish points along different strands.