[TVS diode: See Fig. 2-7(a).]
TVS diodes are typically used under reverse blocking status. (Virtually no current flows and only voltage is applied.)
Breakdown (clamp) occurs only when voltage exceeding a certain voltage (clamp voltage) is applied to a TVS diode.
[Zener diode: See Fig 2-7(b).]
Zener diodes normally used in the breakdown state.
It is assumed that a breakdown (Zener) current always flows in normal state.