When using MOSFET in injectors (solenoids) and igniters (transformers), high voltage which exceeds the maximum rating(VDSS) of the MOSFET occurs as a result of back-electromotive force from L when the MOSFET is turned OFF. If the VDSS is exceeded, current will flow despite the MOSFET being in the OFF state, so this generate high loss and can damage the MOSFET.
How much energy (loss) can be withstood at this time is called the “avalanche capability”.