Typically these are boards that are built for testing a completed electronic component. The are typically very large, many times oversized and can run between 12 to 24 layers or more.
Usually very complex and very tight tolerances and need to be highly reliable. The will usually be used to test thousands of other electronic components.
Not many PCB manufacturers can build ATE boards.
Bay Area Circuits is one that can among a handful of other companies out there.
