Boarding groups are a loading algorithm, not a status ladder

There is a short answer about boarding order and a useful one, and they are not the same. What follows is the useful one.

The problem being solved

A single-aisle aircraft has one aisle and, usually, one working door, so every passenger stowing a bag blocks everyone behind them. Turnaround time is a direct cost to the airline, since an aircraft on the ground earns nothing and delays propagate through the day's schedule.

Boarding sequence exists to reduce the number of times passengers wait behind each other, not to rank passengers by importance. Status boarding sits on top of that as a commercial layer, which is why the two purposes are often confused.

Why back-to-front is not the fastest

Boarding the rear first concentrates everyone in one section of the aisle at once, so each person still waits behind the one in front. Studies of boarding methods have generally found that spreading passengers along the cabin, such as by alternating rows or by window seats first, loads faster than strict back-to-front. Random or open seating also performs well in practice, because it naturally distributes people along the aisle.

Airlines nonetheless retain group boarding because it is predictable, easy to explain and monetisable.

Window, middle, aisle is the physical constraint

Every time a window passenger arrives after an aisle passenger, two people stand up and the row is blocked for an extra thirty seconds or so. Boarding outside-in avoids that entirely, which is why some carriers use it and why it demonstrably speeds up loading. It conflicts with seating families and companions together, which is one reason it is not universal.

The working rule: where it is used, an aisle seat means boarding last, which is the opposite of what most passengers expect.

Overhead space is the real scarcity

A full narrow-body aircraft usually has fewer locker slots than passengers with cabin bags, so lockers fill before boarding completes. This is why priority boarding is sold: it is protection against having your bag taken at the door, not a better seat. Bags placed wheels-first and on their side use the locker efficiently; coats and small bags placed flat waste it.

On carriers that charge for cabin bags, the space problem is smaller, because fewer passengers have one.

Gate-checking and how to avoid it

When lockers fill, remaining cabin bags are tagged and put in the hold, which is free but returns your bag at the belt rather than the gate. Regional jets gate-check almost everything by design, since their lockers are too small for standard cabin bags. The defence is a bag small enough to go under the seat, which no gate agent will take from you.

Anything you need in flight, or that is valuable, should be in a personal item precisely because of this.

Allowances vary by fare class more than by airline.

Boarding early is not always better

Boarding first means more time in a stationary cabin with limited air conditioning and a seat you cannot leave. Boarding last, with only an underseat bag, means less waiting and no locker anxiety. Where you have a tight connection at the other end, a forward seat matters far more than an early boarding position.

The only strong reason to board early is a full-size cabin bag you cannot afford to have taken.

The takeaway

Board late with a small bag, or early with a big one. The middle position is the worst of both.

The trip goes better when the bag is boring.