To be honest I'm having a lot of trouble figuring out what's going on in the circuit.
I'm pretty sure that L4 must be a saturable reactor, and the circuitry around QX is the controllable feedback circuitry for it. The components surrounding Q24 also seem to be part of the reset circuitry, but it's a variation I've never seen before, and can't quite make sense of.... But in normal saturable reactor feedback, the base of QX should be connected to a zener (which defines the setpoint for the regulator) while the emitter is connected to the output voltage. This isn't the case here. The function of the TL431 is the real mystery here. It's really hard to tell without knowing what the "+5T" node is. Does the rest of the schematic show what that node connects to?
- - - Updated - - -
The +12V line must be the main regulated output voltage for the flyback. The 5V rail (and probably the other derived voltages) have saturable reactor regulation, which doesn't require optocouplers, but they do require a voltage reference. But where that reference is generated isn't clear. A more complete schematic is necessary.