The DGA lists
DGACA maintains separate lists by region and type of work. On the West Coast: Southern California, Third Area, Multi-Camera, and UPM Multi-Camera. On the East Coast, a New York list. For commercials, there are New York, Southern California, and Third Area commercial lists. A person is placed on a list for a specific classification, so being on the second AD list doesn't qualify someone as a 1st AD.
The requirements are counted in days of qualifying work in specific jobs, with rules about how much of that can be prep or post versus shooting. The exact numbers differ by list and classification and are published on the DGACA site. There's also a training path: the Assistant Directors Training Program, run with the producers, places graduates on the second AD list.
What it means for a production
A DGA signatory show hiring in Albuquerque checks the Third Area list for its 1st AD and 2nd AD. A PA who has been running the set on non-union shows for years can't be bumped to 2nd AD on a DGA show just because the UPM likes them, unless they're on the list or the list has no one available and the agreement's exception process is followed.
For a non-union AD who wants to move up, the path is documenting qualifying days, which is why keeping copies of call sheets and production reports with your name and title matters. Apply within the time limits DGACA sets after your last qualifying job.
The IATSE version
In Los Angeles, many IATSE crafts use the Contract Services Industry Experience Roster. The work-experience requirements vary by classification, and applicants submit employment verification letters from employers or payroll companies. Placement also requires preliminary safety courses, including General Production Safety and Harassment Prevention Training. Local unions and employers use the roster when hiring.
Common confusions
Being on a list isn't the same as being a member. Qualification makes someone eligible to be hired, and the guild or local handles membership separately.
Lists are regional. Someone on the Third Area list who wants to work in Southern California needs to meet that list's requirements. The DGA page covers how the guild's jurisdiction divides the country, and the IATSE page covers how locals are organized.