(The following is an excerpt from the introduction of the MODIS-AUX - Description and Interface Control Document. The complete document can be found at the CloudSat Standard Data Product Interface Control Document links page. )
The MODIS-AUX data set is an intermediate product that contains a subset of ancillary MODIS radiance and cloud mask data that overlaps and surrounds each CloudSat cloud profiling radar (CPR) footprint. Input data are obtained from the 1B-CPR and AN-MODIS products, and the MODIS-AUX data are used as input to th4e 2B-GEOPROF, 2B-CLDCLASS, 2B-TAU, and 2B-FLXHR algorithms in the CloudSat data processing system. The MODIS-AUX product is created by the Generic-AUX Subset-to-Reference algorithm.
Also see the CloudSat Data I/O Interface Specifications