It is the copper layer for your PCB, for example, if there is one copper layer only, then it is 1 layer.

For now, we only make 1,2,4,6,8 layers.

If your file is 3 layers, we will make it with the process of 4 layers so you need to order it as 4 layers when you place your order.

If your file is 5 layers, we will make it as 6 layers so you need to choose 6 layers.

If your file is 7 layers, we will make it as 8 layers so you need to choose 8 layers.

Note: For 1 layer PCB, there will be one side with solder mask only, if you want to cover it with solder mask for 2 sides, you may need to order it as 2 layers.