In order for TL431 to properly be used as a comparator, the logic output must be readable by the receiving logic device. This is accomplished by knowing the input high and low level threshold voltage levels, typically denoted by VIH & VIL.
As seen in Figure 10-2, TL431's output low level voltage in open-loop/comparator mode is ~2 V, which is typically sufficient for 5V supplied logic. However, would not work for 3.3 V & 1.8 V supplied logic. In order to accomodate this a resistive divider can be tied to the output to attenuate the output voltage to a voltage legible to the receiving low voltage logic device.
TL431's output high voltage is equal to VSUP due to TL431 being open-collector. If VSUP is much higher than the receiving logic's maximum input voltage tolerance, the output must be attenuated to accomadate the outgoing logic's reliability.
When using a resistive divider on the output, be sure to make the sum of the resistive divider (R1 & R2 in Figure 10-1) is much greater than RSUP in order to not interfere with TL431's ability to pull close to VSUP when turning off.