Cologen casings are uasually edible. They are made from the collogen layer just under beef hide, ground up & then extruded to forms for their pre-determined size. other than sizes they come in 2 main types, 1 type for fresh (thinner) & the other that is thicker for smoking that will hold the weight of the product being smoked so it will not tear & wind up on the floor. The larger 2' 3'' etc are usually synthetic casings(non edible).
For breakfast sausage,slim jims I use 22-24mm sheep casings, 24-26 mm is hot dog size. When using smaller diamater collogen casings, using a STRAIGHT funnel not tapered funnel will facilitate stuffing as collogen has no streatch & will tear. DO NOT SOAK collogen before stuffing, but synthetic you will soak for 1/2 hr warm water I am a Sheep & hog gut guy myself, but use collogen & synthetic from time to time. Hope I didn't lose ya