This is a "v" groove cut into the top and bottom surface of an array of multiple PCB's or between a board and rails to be removed after assembly. The cut is usually 1/3 top, 1/3 bottom, leaving 1/3 uncut in the middle. This process is used when removing the tabs of a tab route is not a viable option, this does result in a less smooth finished board edge. The boards are typically set up side by side and end to end with the edges adjacent to each other. After assembly the boards are broken or snapped apart.