.CDF is the extension used by Affymetrix to name files storing information about an array layout. The locations of probes, probe-sets and probe-pairs are registered in this files.

For each type of array, Affymetrix provides a CDF file containing information about its characteristics. If several CEL files belong to the same array type they will use the same CDF file.

Babelomics' data base repository contains CDF files for all standard Affymetrix arrays. If you are using an standard Affymetrix array then you do not need to upload CDF files into Babelomics. At the moment it is not possible to upload custom CDF files into Babelomics so you will not be able, for instance, to normalize custom Affymetrix arrays.