The Air Force is giving airmen in 106 enlisted career fields up to two more years to get promoted before they are made to leave the service under the newest round of high year of tenure extensions.

That represents a slight decline from the 122 career fields made eligible for so-called “up or out” extensions in February 2016, which itself was a broad expansion of the 38 Air Force specialty codes that were previously eligible. The release did not say why the number of career fields was reduced.

The Air Force Personnel Center said in a Monday release that the service is extending tenure limits to hold onto experienced airmen in critical, undermanned career fields, in areas such as intelligence, surveillance and reconnaissance; maintenance; nuclear; cyber; and special operations — areas where the Air Force has been acutely concerned about its readiness shortfalls.

The Air Force’s high-year tenure rules spell out how many years an airman can remain on active duty in a certain grade. For example, a senior airman can serve no more than eight years, a staff sergeant can serve no more than 15 years, a technical sergeant can serve no more than 20 years, and a master sergeant can serve no more than 24 years.

But under the high-year tenure extension program, eligible senior airmen, staff sergeants, technical sergeants and master sergeants in the right AFSCs and grades can apply to have their tenure extended by anywhere from one to two years.

“Although retention is high in some career fields and FY 16 and 17 retention programs were successful, the Air Force needs to ensure experienced airmen are able to complete the mission as well as train new airmen,” Col. Erik Bovasso, military sustainment and transitions programs division chief for AFPC, said in the release. High-year tenure ”extensions will help improve mission capability in key areas where readiness is currently strained.”

The extensions will be effective as of Tuesday, AFPC said. To be considered for an extension, airmen must have been serving in the eligible control AFSC and grade, as listed in the Military Personnel Data System, on July 21. They also must have a high-year tenure date set between Oct. 1, 2017, and Sept. 30, 2018.

Airmen who were approved for a previous extension can get another extension, if they are still eligible, as long as their first extension wasn’t for 24 months. However, their total extension won’t be able to exceed 24 months, meaning if they originally got a one-year extension, they can get another year.

Airmen can submit their applications between Aug. 1 and May 31, 2018.