How heat treatment affects the biscuit flour?
Is this usual aplication or not?
Heat treated flour is denatured so you cannot get any gluten development from it.
If you want to weaken the flour for any reason you could substitute a proportion of your total flour but can't see any other reason why it would be used in biscuits. It is also more expensive than General purpose biscuit flour.
The heat treated flour increases the water absorption of the flour and destroyed the gluten structure so it's not advisable to use for crackers or or semi sweet type of biscuit products .
Using this kind of flour for soft biscuits gives softer eating textures and reduces spread during baking. In my experience you can used this kind of flour for some % for wire cut type biscuits to improve the good shape during the baking .