I working on a SparkFun board. This board has 4 layers but they didn't use layers 2 and 3. Why didn't they use these layers? I installed the design file from Sparkfun. When I generated these files on Altium I encountered some keep out layer in board and connector. These keep out layers overlap and interlaced the top and the bottom layers. Can anyone explains to me what is this? Why we use this layer? Must we use every time this layer? What does this layer indicate?