During the height of iraq/Afghanistan i met few guys called up from the IRR. Not very happy campers. During desert storms the army had such low numbers in a few mos that they pulled guys out of Leavenworth to go then were returned to Leavenworth once they got back. If all this is the prelude to ww3, were gonna be hurting. Just the indiana national guard is at such low numbers its usual the prime sunject in all higher up briefings. Of course they're all itching for a mobilization because it means more money to for the stateI'm glad you asked! Like @actaeon277 said, it depends.
I know the rules can be different for Officers, maybe @Alamo could roll in on the commissioned side.
Individual Ready Reserve (IRR) : These are the folks who did there 4 years and got out. Enlistees have an 8 year service commitment. Do 4-6 years active duty, you still have 2-4 years of IRR before you are done with the service commitment. This is not to be confused with Regular Reservists (which come in many flavors.)
Reserves are Federal (Title 10,) National Guard are dual State/Federal chain of command (Title 32/Title 10.)
A "mobilization" is required to bring anybody back to active duty. These include: voluntary, partial, and full (haven't had a full mob since WWII.) Partial gets Reserve and Guard Units called to active duty for up to 2 years. Partial mob gets the IRR.
During OEF/OIF the Army used the partial mob to keep a lot of troops for 8 years no matter how long their enlistment paperwork said.
IRR = Service calls them to active duty. Full Mob can reach out and get the Retirees (AKA Blue ID Mafia.)
Service commitment: non-medically retired means a 30 year service commitment. I did 21.5 active duty, got letters every year from the Air Force Reserve Personnel Center (since I was Retired Reserve at that point) until I hit my service commitment in 2016. At that point, my service commitment to the USAF was over.
NOT SO FAST, LONE-STAR!! The USAF can't get me, but the DoD can. One section under Title 10 (688 IIRC) divides retirees for recall to active duty into three groups: non-medically retired/retired less than 5 years. Non-medically retired/retired more than 5 years. Medically retired.
One thing I noticed from the letters from the USAF Reserve Personnel Center: it showed me as fully qualified in two career fields.
I was Base Supply for 13 years, and a Manpower and Organization Analyst for 8 years. In September 2007 when I retired (and because of all the crazy rules with secondary AF Specialty Codes) I was still showing as a fully-qualified in Supply Management (if I was active duty in January 2008, it would've dropped off.)
So in 2016, Big Blue (Reserve) still thought I was fully qualified in a career field I left in 1999.
Like Act said, it depends. EOD/TACP/PJ/Combat Controllers are always in demand. Comm (what the civvie world calls IT), no COMM no bomb. AF Cops (Security Forces) will be needed.
Aircraft Maintenance: you ain't gonna put an old F-4 Crew Chief on an F-35, too much training involved. Big Blue will have to play the cards they have.
Civil Engineers: those are are getting recalled.
Supply: Computers have changed. Trucks/forklifts/warehouses? Ain't changed since Nam.