A BackStack of a library is an area that contains resources that are seldom used. Libraries, when they have the space to afford this luxury, ''never'' discard seldom-referenced titles. Instead, they move them to the BackStack where they can always be retrieved if desired.

Think of them as akin to the tertiary store of a three-tier storage architecture.

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I worked a year in the uni's Gift's and Exchanges department. The department was responsible for both accepting and sending out gifts. There were usually about 400 books we didn't want, if other exchange partners didn't want them, we offered them to friends of library books sales, if they didn't want them, they got boxed up and recycled. Now, 99.9% of these books were either irrecoverably damaged or were along the lines of 'Thompkin's county white pages, 1988'. Libraries do indeed keep most of their resources, but they do throw the occasional one out.

P.S. If you ever get a chance to work in a large library, especially behind the scenes, do so.