If you are referring to the active mask generated for IC fabrication which defines the active regions of the chip it can be seen as the places where the think oxide layer will be grown. The thin oxide is basically the gate oxide for MOS transistors.
Hello calliste,
The n+ and p+ regions are doped on the chip at the same time so they have to be doped when you dope the source and drain regions of a MOS. This is done through the gate oxide which is grown before the doping so thats is why those n+ and p+ regions have to be under the active area.