Hi Kwok Ho,
in my system the content repository is stored in ARCH_USR. This is a client independent table. So changes in the customizing in any client should be stored in ARCH_USR and be visible and valid in all clients. I cannot imagine that this should have been changed by SAP.
As I said, if you leave content repository empty but then the files will remain on the server file system. Also the "store" job is not executed because there is nothing to store in.
An external content server is a separate external system like Opentext Archive Server, EASY Archive, Documentum or SAP Content Server. They are built to store a lot of files and guarantee the unchanged long term storage of files.
A Content Repository is a logical unit on an external content server and is defined in OAC0. The Content Repository has to be created also on the Content Server itself. So you will get help from the provider of your content server or in some threads of SCN.
Regards,
Benedikt