Transient protection for a 12V rail — what the ratings tell you
The SMBJ12A933: Its 13.3V minimum breakdown voltage means it starts conducting just above the 12V reverse standoff, shunting surge current before the protected circuitry sees damaging overvoltage. Rated for 600W peak pulse power at a 10/1000µs waveform, this part can absorb a 30.2A peak pulse current — enough for a typical industrial surge on a 12V bus, but not a direct lightning strike without additional series impedance. The 19.9V maximum clamping voltage is the ceiling the downstream components must survive; a 12V-to-3.3V regulator with a 20V abs-max input would be marginal here.
onsemi lists the SMBJ12A933 as Obsolete.
Package and footprint — what to match in a replacement
A pin-compatible replacement must share this footprint and the single unidirectional channel configuration. The supplier device package is DO-214AA (SMB); the mounting type is Surface Mount. No power line protection is provided — this is a signal-rail or low-side supply clamp, not a mains-side protector.