Even if you are very familiar with archival collections, using search terms, and/or filtering using subject fields, it is always recommended to begin with a simple or general search as not all collections and items are tagged consistently. However, if there are more results than manageable, additional filtering options can help.

Depending on the search results, options will appear to show all materials tagged with select terms including the creator of the material, the format of the material (or its digital surrogate), or subject name, topic or place. The number next the term indicates how many items are tagged with that term.
Note: This does not mean that every item in the collections is tagged with every possible term, and while filtering using terms such as "United States. Army - Cavalry (2)" can help narrow a search, it can also leave out many potentially useful results that have not been tagged with that vocabulary or are in a collection that has not been digitized.