This is interesting. Just found this on delta.com. I had no idea that Main Cabin boarding zones were divided by fare class, and that T, X and V come after higher ones, and that this is in addition to zone 4 BE boarding. I assumed that after Zone 1 it was done back to front of plane.
This link says:
Zone 2: Main Cabin Passengers
Zone 3: Main Cabin Passengers booked in T, X and V fares
Zone 4: Basic Economy Passengers (E)