From UA Libraries Digital Services Planning and Documentation
What is an object folder?
Each object that we digitize has a number assigned to it (such as u0001_0000001_0000001). We add an extra set of numbers to that number for the scan or scans (such as u0001_0000001_0000001_0001), numbering consecutively in those last four digits for multiple pages. When the object has more than one page, it is stored in a folder labeled with the 3-part number, with each page stored inside it with a 4-part filename.
Example: In the Wade Hall sheet music collection, the piece "Naughty Marietta" has been given the digital number u0004_0000002_0000331. The folder for this piece is labeled with that number, while the individual pages are labeled u0004_0000002_0000331_0001, u0004_0000002_0000331_0002, etc.