PCB Shape layer (also call outline layer). Similar software protel, commonly used "keepout layer" to define the a PCB shape layer. Some people also like to use mechanical layer, these are not problems.
We do not recommend that you use the silkscreen layer to define a PCB shape layer.
Normally, we think of some PCB shape such as the following (sharp shapes, fine rectangular shape, easily broken). May result in transport packaging or processing difficulty.
We may try to avoid similar PCB shape as following: