Word 2003, XP, + all updates

It is possible to define your own caption label text (insert -> reference -> caption -> new label).

These captions appear to be bound to the computer and/or user, not the document.

If you create a document with these own defined captions ....
1) somebody else logged onto the same computer who then opens the document will see the captions that have been created, but will not be able to create any additional captions using the same label.
2) similarly, somebody else or yourself using another computer who then opens the document will see the captions that have been created, but will not be able to create any additional captions using the same label.

Does anybody know how/where the own caption label information is stored by word, that would explain these situations?

Any help would be much appreciated.