The way Delta decides how you board reveals a huge problem in the airline industry (DAL)

Many airlines' boarding processes are complicated because of the number of customer categories and ticket classes.
When airlines experiment with new boarding strategies, they address only part of the problem by focusing on boarding lanes and reorganized boarding groups.
Outside of active US military personnel and passengers who need assistance,

As airlines create new ticket classes and rewards programs, it can be hard to decipher the distinctions among boarding classes. Looking at an airline's website reveals how complicated the boarding process has become.

Airlines like American, Delta, and United have experimented with new boarding strategies designed to make the process more efficient by focusing on boarding lanes and reorganized boarding groups, but that's only part of the problem. No amount of reorganization can mask the notion that airlines are creating too many ways to divide their customers.

Outside of active US military personnel and passengers who may need special assistance, United uses 19 categories to separate passengers into boarding groups. American has 22, while Delta tops them both with 27.

These are the categories Delta uses to separate passengers into boarding groups and what they mean, starting with passengers who get to board first.

Premium boarding zone

Delta One passengers:
First-class passengers:
Diamond Medallion members:

Sky Priority boarding zone

Platinum Medallion members:
Gold Medallion members:
Delta Comfort Plus passengers:
Flying Blue Platinum members:
Flying Blue Gold members:
Virgin Atlantic Flying Club Gold members:
Virgin Australia Velocity Platinum members:
Virgin Australia Velocity Gold members:
GOL Smiles Diamond members:
SkyTeam Elite Plus members:

Zone 1

Silver Medallion members:
Delta Corporate passengers:
Priority Boarding Trip Extra passengers:
Reserve Delta SkyMiles credit-card members:
Platinum Delta SkyMiles credit-card members:
Gold Delta SkyMiles credit-card members:
Flying Blue Silver members:
Virgin Australia Velocity Silver members:
GOL Smiles Gold members:
SkyTeam Elite members:
Crossover Rewards SPG Platinum members:

Zone 2

Main-cabin passengers:

Zone 3

Main-cabin passengers booked in T, X, and V fares:

Zone 4

Basic-economy passengers:

A prior version of this story incorrectly said Delta has seven boarding groups, outside of those used for active US military personnel and passengers who need assistance. Delta has seven total boarding groups, but only six for passengers that are not in the US military and don't need assistance boarding.