[1] Materials housed in special collections can be in any format (including rare books, manuscripts, photographs, archives, ephemera, and digital records), and are generally characterized by their artifactual or monetary value, physical format, uniqueness or rarity, and/or an institutional commitment to long-term preservation and access.
[3] Some special collections are recognised as being of global importance; the UNESCO Memory of the World Register includes documentary heritage material.
In contrast to general (or circulating) libraries, the uniqueness of special collections means that they are not easily replaced (if at all) and therefore require a higher level of security and handling.
Depending on the policies of an institution holding special collections, researchers may be asked to present identification cards, letters of reference, or other credentials to gain access.
The 2010 "Taking Our Pulse" report cites a survey in which 67% of responding institutions use offsite facilities, with another 5% in planning stages.