Plain English Meaning

An ATCAA is a temporary chunk of high-altitude sky that air traffic control sets aside for a specific activity, usually military training. While it is in use, other aircraft are kept out of it. When the activity is finished, the airspace is released and normal traffic can fly through it again.

Full Definition

An ATC Assigned Airspace (ATCAA) is a block of airspace, defined by horizontal and vertical limits, that air traffic control assigns for use by a specific user — typically the military — to contain activities that must be separated from other instrument flight rules (IFR) traffic. ATCAAs are established above Flight Level 180 in airspace controlled by ATC and are activated only when needed. While active, non-participating aircraft are routed around them.