TL431 Constant Sinking current
Perhaps you miss-understand the use of the TL431 and how it works. At heart it is a shunt type voltage regulator, an adjustable zener diode. You supply a safe current through a resistor from a voltage source that is within the TL431's safe limits. By attaching resistors or a trim pot wiper to the control pin, and the CW and CCW pins to cathode and anode, the cathode becomes a multiple of the ratio \$(1 + R1/R2{\cdot} 2.50)\$.

By itself it can only be an adjustable voltage reference, but a very stable one. To get a current clamp or constant current sink you need a transistor to form a feedback loop. Your pdf of this part shown them in detail. Anytime you have a resistor (Re) from the control pin to the anode the cathode remains as positive as possible until the transistor (NPN) conducts enough current so that the Vdrop across the emitter resistor (Re) equals the 2.5 volt internal ref voltage of the TL431. The current is fixed or clamped at 2.5/Re.

This same circuit can act as a current clamp because it will not allow more current to flow at the NPN collector than what creates the 2.5 volt drop on the Re resistor. The current is 'locked' even if the voltage rises up to the safe limits of the transistor, which could be a 300 volt MJE340.

The NPN could be replaced with a N-channel MOSFET with a 700 volts rating. Note that the Collector/Drain voltage can be separate and much higher than what the TL431 needs to function ok. To control a MOSFET the TL431 needs a 12 to 15 volt source so the MOSFET gate can reach the +10 V saturation voltage.

There are many examples for using the TL431 in the datasheet, often using external transistors to boost or limit the current supplied to a load, as well as controlling the supply voltage.

Precision resistors are often required not just for accuracy but because they are very temperature stable, so the voltage/current you set is the same hours later, to .1% or better. Beware of large value trim pots, as they have a temperature drift of ~200 ppm C.