It's a lot easier to understand our archival collections interface if you know something about archives in general -- what kind of materials they contain and how they're structured. If you're new to archival research, this page should give you a good grounding.
According to the Society of American Archivists, archives are
records created or received by a person, family, or organization and preserved because of their continuing value.
This generally means unpublished items, most of them unique.
But, especially in the modern era, these collections many contain things that aren't entirely unique, like duplicates created with carbon paper or a copy machine. They may also include ephemera that was mass produced, like theater programs, pamphlets, and postcards.
Sometimes, these collections also contain published items. While these are generally removed for cataloging at our institution, they are sometimes left in the archival collection. For example, a family Bible is of more use with the family's papers than in the general research collection.
Of course, to get to that point, they had to be created in the first place, preserved, and seen by someone as having value. A lot depends on the social position of the materials' creators, so there are many ways that this process can go wrong, leaving gaps in the historical record.
To explore this reality of archival research, at least from an archives/libraries point of view, see David Thompson, Simon Fowler, and Valerie Johnson, The Silence of the Archive (ALA Neal-Schuman, 2017).
The box is the basic unit of the archive. It's what you request to see during a visit to the reading room, even if what you want to look at is one folder or even one item!
Arrangement of materials is both intellectual and physical.
Most times, the physical follows the intellectual -- that is, the grouping and order of materials translates directly to their order in folders and boxes.
But this may not always be true. For example, materials that are added to a collection after it's processed may not be housed in a box with like materials from the initial donation, even if they are grouped with them intellectually. Or say a collection contains an oversized item or artifact -- it may be housed in a box with like pieces, even from other collections.
What is important is that a Finding Aid will always tell you what box the materials are in, even if it's not the one you expected.
Note: At our institution, multiple collections may be combined in one box if they are all small. We may also give you access to materials that have been minimally processed -- that is, they are still in their original folders or not in folders at all. In those cases, we may not be able to provide much information about what is in each box.
1. Accession -- Formally accepting materials and taking physical and legal custody of them
2. Arrangement -- Organizing materials in a logical way that preserves their original order as much as possible (which may be important!)
3. Description -- Creating a Finding Aid or other guide that let potential users know what's in the collection
Archival Collections is that guide for our materials -- it is a searchable database with information about collections, which it can also transform into Finding Aids for you to download.