Casing in sewing construction is a fabric “tunnel” sewing into a garment or sewn item to hold boning, elastic, cording, etc.
Casing can be built into a pattern. An example of this is folder over elastic casing in a waistband.
Casing can be added nearly anywhere using a fabric tube, ribbon, bias cut fabric, etc.
Boning casings are made from strong fabrics – such as coutil.
Elastic casings are often times made with lighter fabrics or soft ribbon to reduce bulk.