After the last water during the incubation in the casing soil, the grower wants the mycelium to grow onto the surface.
To ensure a good recovery of the mycelium in this period the right settings for the room climate are essential.
An RH at a minimum of 95%. A very slow air speed, if possible zero and an adequate room temperature. That room temperature should be as close to the optimum for vegetative growth as possible. That optimum temperature is 25° C to 26° C.
This means often a room temperature of around 21° as a minimum.
Then the mycelium has the right conditions to grow onto the surface.
The next question is: how high do I let it grow ?
Higher means better control of the number of primordia, so no surprises about the number of mushrooms growing out. Higher also means easier picking so a higher pick rate. Another positive experience is that clustering is easier avoided.

But the big dis-advantage of having the mycelium higher on the surface is that the climate conditions have to be perfect. The influence of the climate onto the mycelium and the primordia is bigger. So one moment of for instance too low RH means that you will lose primordia.
Deeper mycelium is also easier to stagger the mushrooms so that is an advantage too. Another point is the fact if the grower can stop the mycelium when it grows too high. Overlay as that is called makes the outgrow of primordia more difficult. They come from underneath the blanket of mycelium.

So what determines the choice?
Both ways have their pro’s and con’s.
But for me the crucial point to choose a way is the technical capacity of the farm and the experience of the grower. If you are able to stop the mycelium at all times, still create the stagger and have perfect control of the environmental conditions, grow white.
If you are not sure of all this, grow deeper into the surface of the casing soil. So grow blacker.
Generally blacker is safer.