I have no qualifications to answer this since I have never cupped or sold a worm in my life. But I will give it a stab and if I get it wrong somebody will correct it. When worms are sold for fishing in small quantities like a dozen in a "cup" the worms need something to live in or bedding. The qualities of this material are it needs to hold moisture well and be fluffy. Vermicompost is usually not used for this. The material also does not seem to need food. From buying fishing worms in cups as I remember the material, this "cupping" material has varied from pebble sized dark claylike material to a peatmoss or stringy wood type material. I think the cupper or worm shipper uses whatever material they have around that is cheap and available that will hold a bit of moisture yet remain fluffy.