Audiovisual material at the ATM falls under four broad categories:
F – field collections. Collections and items that were gathered by researchers in the field of their work (usually in a specific geographic location, or with a particular cultural group).
B – broadcast collections. Recordings that were either recorded off of the radio or television by a researcher, or materials originally produced by a broadcast company for broadcast.
D – demo collections. Recordings made of multiple, various recordings used by faculty and scholars to demonstrate something, usually in a lecture or seminar, or that were used as early course reserves.
C – commercial items/collections. Commercially released and published recordings. Please note: commercial items are NOT considered primary sources in most cases.
An example showing where the category is located in an archives call number:
This broad category (Field, Broadcast, Demo, Commercial) is reflected in each item's call number. For example, the image below shows the catalog record for a 78rpm recording of Hoagy Carmichael's "Georgia on my Mind."
Under the "Holdings" information, the call number begins "86-231-C"
The "-C" indicates that this item is a commercially released recording

If you see a number after the letters "ATL" in an ATM Call Number, that means there is an access copy for the recording on-site in the ATM Library. So if you wanted to listen to the above item, you could come listen to it on an audiovisual playback machine in the ATM Listening Lab.