You might want to read the following articles that were published in 1989 and 1990 in the proceedings from the annual meetings of the Association for Living History, Farm and Agricultural Museums. If you can't find the Proceedings in a library near you, you can order them online, here:
Deborah Scott, "Tier Levels and Collections Management: Adapting Traditional Museum Approaches for Historic Site Use," 1989 ALHFAM Proceedings
Carl Schlichting, "Tiering Collections: From a Conservative Point of View," 1989 ALHFAM Proceedings.
Tom Brown, "Collections Use in Living History," 1990 ALHFAM Proceedings
All three of them have to do with the concept of tiering collections, which is much like what Dani described in terms of what is not expendable under any circumstances, down to what is freely expendable.
I hope this helps.
Martha Katz-Hyman
Curator
Jamestown-Yorktown Foundation
Williamsburg, VA