The term "casing" as it is used in the mushroom cultivation is the method by which substrate (either PF style cakes or colonized grain or colonized compost or dung or straw.
The term "casing" as it is used in the mushroom cultivation is the method by which substrate (either PF style cakes or colonized grain or colonized compost or dung or straw...) is crumbled into suitable pieces if necessary, and covered with a non-nutritive layer such as peat, vermiculite, coco coir or different mixtures of the previous with various additives.
Casing is a simple process: You layer and level the substrate to an appropriate container and cover with your casing layer.