Southwest Airlines has generally been known for its KISS (Keep It Simple, Stupid) travel rules. Simplicity has, over the years, done much to endear the airline to travelers.

Even as Southwest has followed other carriers into the add-on fee maze, the airline has largely kept it pretty simple. Until now. Their latest boarding offer has even some travel industry veterans confused.

After several phone calls and visits to the website, here is an attempt at an explanation:

For years, Southwest has had A, B and C boarding groups. (The “C” as many passengers say, stands for Center). Being in group A usually guarantees a decent seat and overhead space.

Originally, travelers could get into the first group simply by checking in as early as possible, starting 24 hours prior to flight time. (Which meant many travelers would set their watches, or have an administrative assistant set THEIR watch, to make sure they got the A boarding pass.)

Like many airlines, Southwest then started giving priority to its most frequent customers. So “A List” members automatically get into the first boarding group.

Then the airline started offering EarlyBird check-in for $10 each way, which allowed passengers to be automatically checked in more than 24 hours in advance. The fee has been popular, and in fact will soon be raised, reportedly to $12.50.

More recently, Southwest came up with “Business Select,” a higher priced fare that is completely changeable, comes with free drinks and, most importantly for many frequent fliers, guarantees a boarding pass numbered from A-1 to A-15, which means they will be the very first 15 people to board the plane.

For most travelers, the system has been working without too many headaches. Purchasing EarlyBird check-in doesn’t absolutely guarantee the A boarding group (in case of a flight with a full Business Select section, and more than 45 elite fliers), but I’ve not had a client complain yet.

But the latest wrinkle in the boarding process may or may not be the one that really damages Southwest’s customer friendly reputation. It’s another new early boarding fee — $40 for “Priority Boarding.”

This new $40 fee cannot be paid in advance and will only be available for sale at the airport, if Southwest has not sold all their “Business Select” seats. When available, 45 minutes before departure, paying this fee will allow passengers to join the first A 1-15 group.

Does this new fee and policy mean people who don’t pay the extra $40 won’t be able to get an aisle seat? Probably not. But it does mean that Business Select and Priority boarders will probably nab the exit row seats and the much coveted seats in the first rows.

While Southwest for now will only allow up to 15 in this early boarding subgroup, it’s hard to imagine if the concept takes off, that the airline won’t be tempted to expand it.

Then, what happens if it reaches the point that checking in exactly 24 hours in advance, or paying a regular Earlybird check-in fee, still means potentially ending up in a center seat?

Airline passengers are a resilient lot, but since Southwest has made its reputation on no fees and egalitarian customer service, I have to wonder: how long until many of its fans “want to get away” to another airline?