MCX thoughts on casing layers:
A casing layer is a layer of material applied on top of the colonized substrate in mushroom cultivation. I consider psuedo casing layers to be applied as last step in spawn2bulk (s2b) process and casing layer to be a layer of some composition of substrate applied loosely as an initial step into fruiting (where its applied once surface colonization is visible covering most if not all of the surface).
Casing layers generally can help improve yield, reduce certain types of surface contaminants, and maintain humidity around the fruiting bodies. Conversely they are another variable added to contamination potentiation and can promote contam in some instances (like if prepared incorrectly be it wrong hydration conditions or composition of nutrients or improper pasteurization or sterilization, etc)
In my experience, using an organic sphagnum peat moss mixed with lime powder and filtered water has produced successful tubs without the need for pasteurization or sterilization. I just make the mix as part of my s2b and apply psuedo casing as last step after incorporating my spawn and substrate homogeneously, compacting and edging firmly, and placing a thin layer of colonized grain sprinkled over the cake, and that is then pseudo cased with my peat lime layer and put into colonization area.
While some species require a specific type of casing layer after substrate colonization, most do not require one. However, adding a casing layer can be beneficial for improving yield and humidity control.
Personally, I have used a casing layer when colonizing pasteurized wheat/rye straw to provide an even fruiting surface for mycelium. I have also experimented with an organic peat and lime dust casing layer (no pasteurization or sterilization), which has helped retain humidity and has not resulted in any contamination. Conversely ive experimented using padtuezied wheat straw as a casing layer after cvg tub has colonized. It worked! Although it provided uneven surface for pins to anchor off of, so many fruits that pinned and matured off of the colonized psuedo-straw-casing: fell over (and because of inconsistent spread of the straw material itsef, some pinning occurred on the surface of the cvg cake some in tandum with the straw pins and some shortly thereafter!
Personally, I've worked some APEs in that past that had a casing layer applied in the same instance as the S2B occuring, essentially the compacted bits colonized faster than the looser casing layer. Where I always use casing layer, is if I use spawn to colonize pasteurized wheat/rye straw, if only to provide an even fruiting surface for the mycelium! But, even then... most cubensis can fruit solely on pasteurized straw as the substrate, with no casing layer!
Whether or not to use a casing layer is largely a matter of preference. While it may not be necessary for some species, it can improve yield and help maintain humidity. Experimenting with different casing materials and application methods can lead to successful fruiting and a deeper appreciation for the wonder of fungi.