1.24 V reference threshold — sets the low-voltage floor
The ±1.5% tolerance is tighter than the standard TL431's ±2% grade, which buys you a few percent more margin in the regulation band without adding a trim pot.
15 mA output, 80 µA cathode — load vs. standby trade-off
The 15 mA output current is enough to drive the optocoupler LED in a typical isolated feedback path or to bias a small-signal transistor. The 80 µA minimum cathode current sets the lower bound for the bias resistor — design the resistor so the cathode current stays above 80 µA across the full input voltage range, or the reference drops out of regulation. That 80 µA floor also keeps standby power low enough for always-on auxiliary supplies pulling under 10 mW.