The casing generally indicates the section of the intestinal tube of pigs (but also of cattle, horses, and sheep), carefully worked to be used to contain meat mixtures.
The casing was, and still is today, emptied of its contents, carefully washed and left to dry (through numerous saltings) to be used as a casing to enclose the mixture of sausages and allow them to mature.
There are various types of casing that take different names depending on the part of the intestine from which they are obtained: twisted casing, the gentile, the crespone, the cresponetti, the filzettoni, etc.
Other parts of the digestive system could be used as casings to contain meat mixtures or as containers for other types of food: the pig bladder, for example, was used to produce raw (culatello) or cooked salami or to preserve lard.