How to add Mouse Bites or V-Groove to PCB In Altium?
I had one project with an L shaped board. The PC assembly house wanted me to keep the inner corner of the board in place so that it was better supported by the rails through the pick and place machine. So I used mouse bites to connect the two sections together but to later have that inner corner removed when the boards went to functional testing. 

Here is a picture showing the board (red outline) and the breakaway inner corner (green outline). 



Here is the detail of the mouse bite design from the board fabrication drawing.



Notice that the breakaways are used in conjunction with routing that is performed using a cutter bit that is 100 mils in diameter.