The client 000 is untouched and kept as standard at any point of time.where as 066 is used as early watch client and 001 is used by many users where data might get changed frequently.
one more important feature is cross client object changes" options set to "changes to repository and cross client objects are allowed" whatever the changes you do in 000 it will effect the remaining. while the reaming clients are set to "No changes to repository objects are allowed"
why we use only DDIC ?
DDIC and SAP* are special and they remained untouched they will not get updated by any customizations and don’t have Authorizations issues. SAP* is suppose to use for first time login where as DDIC is the user who can change DDIC objects and owner of objects.so
Cheers,
Manoj.R
