The latter is required because archival material in most cases consists of unique, unpublished records which are available in only one place (unlike books or other published material held at more than one library)
In a bibliography you should provide the full citation details for each archival record/collection you refer to in your essay etc. For example:
Wollongong University Archives: Francis McCaffrey; D92, Francis McCaffrey Collection, 1865-1932; D92/5, Notebooks.
If you refer to the same citation more than once in your essay etc. you may abbreviate the citation the second time rather than write it again in full.
The abbreviation for the University of Wollongong Archives is NWUA (the N denotes 'New South Wales'), so the abbreviation for the above citation would be:
NWUA: McCaffrey; D92/5, Notebooks.