SBIT & PBITs are the ones that run automatically, with SBIT running automatically at startup and PBIT running on a watchdog. The SBIT time budget and scope is usually much smaller than IBIT so time intensive tests like ones that talk to sensors on the bus aren't present in SBIT. You can think of the stages as SBIT: can I run? IBIT: should I run? PBIT: am I running right?