The reason for both is that the 'Master' list is stored permanently, so that it's available for copying into any document without the need to recreate the references from scratch for each new document.

As for "for each reference I do work equally", you'll find you cannot insert a citation to a reference that exists in the 'Master' list but not in the 'Current' list. Likewise, if you get a document from someone else and that document contains some references in its 'Current' list that you'd like to use for your own purposes, you can copy them from there into your 'Master' list, from whence you can then copy it into any document you choose (you could also simply copy the citation to another document, but that wouldn't add it to your 'Master' list).

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]