In the section on the ZMP point it was mentioned that if the ZMP is at the edge of the foot area or beyond it, then the foot will tip over and likely fall. The support area is the generalization of this concept to two feet.
The ZMP can exist anywhere inside the polygon formed by the two feet without the ZMP of either foot leaving their respective foot areas.
One useful thing to keep in mind is that under zero acceleration conditions the COM must be inside the support polygon otherwise the robot will fall because the ZMP will be outside the support area.
This follows from the one mass model based ZMP equation
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