I always thought that a Bunker was more of a small defensive structure that contained with a squad of infantry or a machine gun, and a pillbox had any type of antitank weapon in it, but is there an actual official definition of it.
This is all I could find.
They are different.
In their simplistic difference:
pillbox: able to fight from it, small in size
bunker: living quarters, larger in size
Bunkers had a common design, as did the pillboxes. Bunkers also had a machine gun firing slot that covered the entrance to the bunker.
See The German Army Handbook, published by the University of La. It has decent figures of bunkers and pillboxes in it.
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