If you “Limit to collections”, the search will return a list of collections that contain your search term somewhere in the Finding Aid.
If you select “Search all record types”, the search will return a list of series, boxes, folders, or items that contain the search term in the description.
You can limit your search to a date span. Sometimes, archival material is listed according to the date it was created, not to the time period in reference.
For example, oral history interviews with Vietnam War veterans are dated to when the interviews were conducted, but the content covers their experiences. A search of “interview Vietnam veterans” to the 1965-1975 time period would not return the interviews.