Alien crew associated with this alien race (e.g. race units + terrorist units), defined by a list of unit string IDs. Order here will define rank in terms of deployment. Generally speaking the order is: 0 - Commander 1 - Leader 2 - Engineer 3 - Medic 4 - Navigator 5 - Soldier 6 - Terrorist 1 7 - Terrorist 2but of course, not all races have all ranks, so the "fill in" rank would be listed multiple times (see Ethereals).
Alternative to members. Instead of defining just a single unit type per alien rank, you can define any number of unit types here and one will be chosen randomly (each time a new unit is generated). To increase a chance of a certain unit type being selected, you can add it multiple times.