Who it covers on a feature
A lot of people outside the business assume the DGA is only for directors. On a union feature, the guild's jurisdiction usually includes the director, the unit production manager, the 1st AD, the 2nd AD, and any additional 2nd ADs. That's the group that builds the schedule, runs the set, and signs the production reports, so DGA rules shape how the day runs even for crew who aren't members.
How the guild affects hiring
Signatory employers under the DGA Basic Agreement give preference of employment to people on the Qualification Lists for UPM, 1st AD, and 2nd AD work. The lists are administered by DGA Contract Administration and split by region: Southern California, a Third Area list covering most of the country outside Southern California and New York, a New York list, and separate commercial lists. A production in Atlanta hiring a 1st AD will look at who's on the Third Area list before going elsewhere.
Independent productions
Smaller films usually sign under one of the DGA's low-budget agreements, which scale salary minimums and staffing requirements by budget. The DGA low-budget agreement guide covers how the tiers work. Signing still means paying pension and health contributions, following DGA rules on things like creative rights and credits, and in many cases committing to residuals if the film sells into later markets.
What producers get wrong
Hiring a DGA director and then assuming the ADs can be non-union. Depending on the agreement, the production may have to staff DGA assistant directors and a UPM as well. Read the staffing rules for your tier before making crew offers.
Budgeting only the salary. DGA fringes (pension, health, and in some cases vacation and holiday pay) sit on top of the rate. They go in the budget's fringe section, and underestimating them is a common reason a low-budget top sheet goes over. The film budget fringes post shows how to calculate them.
Missing prep and wrap weeks. A 1st AD's deal includes prep time before principal photography, and the DGA agreements set minimums for it. A schedule that has the 1st AD starting the Monday of shooting doesn't match any real agreement.
Relationship to other unions
The DGA sits alongside SAG-AFTRA for performers and IATSE and the Teamsters for crew. Each has separate paperwork. A production can be DGA and SAG-AFTRA signatory while hiring non-union crew, though union crew members may not be able to accept that job.