Instead of merely converting an input value, the ADC can be configured to compare the result to a given value and only issue an interrupt or event when the result is above or below that value. Interrupts on compare match (above/below) can be configured individually on each channel, but the compare register is shared between all four virtual channels.
Typical use of this feature is to leave one or more ADC channels in free-running mode and configure the ADC to issue an interrupt when one of the input signals reach a certain threshold.