Brittany, I was struggling with this same issue. Then in a PastPerfect Cataloging Collections Webinar this past May, Brian Gomez said this:
“Archives is unpublished work, Library is published work, Object is three dimensional, and Photo is two-demensional.”
The PastPerfect Newsletters are very helpful and that is how you find out about upcoming Training Webinars. You can sign up to receive the newsletter here: http://www.museumsoftware.com/newsletter.shtml .
You can also find out about upcoming PastPerfect Webinars on their Facebook site at: https://www.facebook.com/PastPerfectSoftware .
And if all else fails you can always call or email them: http://www.museumsoftware.com/contactus.shtml .
They are always very willing to help with any questions or concerns that you may have.