As I said before, the only references you can cite are those that are in the Current list. You cannot cite something from the Master list without first adding it to the Current list. Whenever you add something to the Current list via the 'New' button, it gets added to the Master list as well - which is why you won't ordinarily see any difference between their entries. The only times a difference in the details might crop up is if:
• you copy a citation from another document into your current one and the source has a different version of the reference to one that's in your Master list;
• you edit something in one list and not the other (Word prompts to update the Master list if your edit the Current list, but not vice-versa); or
• you add a new entry to the Current list for which there is an existing entry in the Master list (again, Word prompts to update the Master list).

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]