Southwest Airlines has four different boarding groups: A, B, C, and pre-boarding, and the airline assigns boarding groups and positions (1–60) based on check-in time, fare type, and elite status. Passengers who check in earliest get better positions. Moreover, the passengers of Group A can board the flight first, followed by Group B and then Group C. Further, the passengers who came under the pre-boarding category can board the flight before all three groups. To know more about how does Southwest determine boarding groups, visit its official website or read the blog at the mentioned link.