The SMAJ20CAE3/TR13: The clamping voltage at the 15.4A peak pulse current is 32.4V, which sets the upper limit the protected circuitry must survive.
Breakdown and clamping — where the guard band lives
The 20V reverse standoff means this diode will not conduct appreciably below that rail voltage; leakage stays low. The 22.2V minimum breakdown gives a 2.2V margin before the device starts clamping — enough to absorb normal ripple on a 20V supply without wearing the junction. At the 15.4A peak pulse current the clamp hits 32.4V maximum. That 12.4V above the standoff is the dynamic resistance of the Zener structure; a downstream DC-DC converter or linear regulator must have an input rating above 32.4V or be protected by a second stage.
Power line protection is not provided — this is a signal- or low-power-rail device. The single bidirectional channel clamps both polarities, which is useful on AC-coupled data lines or bipolar supplies where a unidirectional part would need two devices back-to-back.