It is a cotton fabric whose treads are regular (so as the linen) and just for this feature it is used to embroider on those fabrics whose thread is not regular. It corresponds to 40ct linen.
You can put the "cencio di nonna" on the fabric you want to embroider and when you finish your work you can simply unthread the "cencio di nonna", obtaining in this way a very precise job.