I am reading the book “Kicad Like a Pro, 2nd”. Inside the book is an example ( Project 3 ) where to use freerouter with a 4 layer board. The layout has two net classes. The default and the second called power which includes Vcc and GNDPWER. When I select the power type for layer b.cu then this layer is not displayed in Freerouter anymore. What is the reason for this?