The CLASS developer team provides some flexibility about the CLASS data format through a customizable user section mechanism. This user section can be encoded and decoded only by external programs linked with the CLASS library and which follows the CLASS API. This is the responsability of the external programs to ensure the efficiency of their codes. We recommand to observatory who would like to use this mechanism to contact us (gildas@iram.fr) in order to discuss how to maintain the long term compatibility of their user section.