The voltage at CFB is compared internally against the bandgap voltage (1.23V) if the voltage drops below the bandgap the CFB voltage is used instead in the current setting control circuitry.
So the current setting will reduce proportionally with VCFB when VCFB < VBG.
This feature can be used to directly control the current setting and act as a dimmer input by controlling the voltage at CFB, or when CFB is tied to the input by a voltage divider it will drop the current setting if the input voltage falls low enough that VCFB < VBG, such as if the current setting being too much for the supply.