Auxiliary data are all other data files used to generate a product, other than the direct measurements of the instrument.
Auxiliary data include calibration data that was measured on-board but are not part of the main measurements of the instrument; external calibration files from sources other than the satellite; processor configuration files, and any other files needed by instrument processors.
Envisat and ERS Auxiliary data is shown below.
Envisat Auxiliary Data
How to Access EO Data