The masks show how critical alignment is in metal gate transistors. The green, magenta and light blue have to just touch. Too much overlap or too far apart and

The masks show how critical alignment is in metal gate transistors. The green, magenta and light blue have to just touch. Too much overlap or too far apart and you don't have a working transistor. With polysilicon gates the equivalent of the green would be one big rectangle, but since it would come after the gate (instead of being the first step like here) it would actually become two separate rectangles just touching the gate on each side.