What is the diffirence of silicon unilateral switch and silicon bilateral switch?

SUS - Silicon Unilateral Switch

is a type of thyristor used as a breakover device that conducts current in only one direction, it has a third terminal that is used to alter the breakover voltage if connected to a zener diode.

SBS - Silicon Bilateral Switch

is a type of thyristor used as a breakover device which is capable of triggering triacs, it has a lower breakover voltage compared to triacs.