When you sign you initial contract with the military, you do x# years active or reserves, and x# years inactive ready reserve. Equal to 8 years total. When in inactive ready reserve (IRR) status. You are supposed to keep the gov informed of your number and such at all times. Most never follow this, but it isnt too difficult to track a person down unless they want to hide. For the remainder of your IRR years, you can be activated at any time, for any period of your remaining years and any amount of time they deem required after (think stop loss around OIF/EIF), and must still adhere to certain standards. Like no drugs. This case, he is calling back up to 450 IRR people and the rest reservist activations to equal 3K people added to the Atlantic EU command... Basically supporting ukraine with troops through manpower to rearm/repair equipment at least.