The anti-aliasing filter is a lowpass filter meant to attenuate signals above the Nyquist frequency to prevent aliased signals. The 4303 automatically sets this based on the sampling rate. It uses a combination of analog and digital filters to achieve this, as documented in the User Manual
The IIR filter is a user configurable digital AI filter
that can be added to an input in addition to the anti-aliasing tracking filter. It can be useful to add in environments where there is excessive high frequency noise that is below the anti-aliasing filter threshold.