TAMPA, Fla. (WFLA) — Southwest Airlines is now limiting the number of EarlyBird Check-In spots that are available on certain flights.

As of Aug. 15, EarlyBird Check-In is now “subject to availability on certain flights, routes, or days, meaning EarlyBird Check-In is unavailable for some Customers looking to purchase it,” according to the airline.

The airline said the decision was made as the airline works on “product enhancements.”

“Many initiatives are underway across the Company to modernize the Customer Experience and win more Customers,” the airline said in a statement to WFLA.com. “This focus brings an opportunity to evolve our fare and Ancillary Products, while maintaining the value they bring to our Customers. Similar to our Fare Products and Upgraded Boarding, EarlyBird Check-In might not be available at any given time.”

EarlyBird Check-In allows customers to pay, starting at $15, to be automatically checked in to a flight 36 hours early, reserving them a more favorable boarding position.

Southwest has three boarding groups — A, B, and C, and passengers in each group are assigned a number, 1 through 60, with A1 boarding first and C60 boarding last. Passengers who don’t pay for EarlyBird Check-In are assigned a boarding position based on the order they check in starting 24 hours before the flight.

The higher boarding positions are coveted by those who want their preferred seat (or those in a travel group who want to increase their chances of getting to sit together), as Southwest seating is first-come, first-serve rather than assigned seating.

If EarlyBird Check-In is unavailable for purchase, passengers can guarantee an A1-A15 boarding position by purchasing a Business Select ticket, which can cost significantly more money than the base “Wanna Get Away” fares. Business Select fares also include free in-flight Wi-Fi.

Passengers can also wait until the day of their flight to upgrade to an A1-A15 boarding position starting at $30, if there are any spots left open.

Southwest Airlines also recently announced that all fares now include same-day standby for free, excluding any potential taxes and fees. Same-day standby allows passengers to keep their current ticket, but be placed on a standby list for an earlier flight in the day, if there’s a spot available.