The main AutoYaST control file (default) is stored in /data/autoyast/xml. All class files must be located in in /data/autoyast/xml/classes as this directory name is hard coded in the AutoYaST plugins (see Class Files).
We do not use any class files for the installation of SLES systems. For OES systems our solution uses class files to query the passwords for the OES installation user, for the AD Administrator in case of NSS-AD, and for the DSfW Administrator.