Originally Posted by NickB

Let us try again, simplifying (at the cost of accuracy ):
the "controlled area" is airside. A "decontrolled" passenger is a passenger that leaves the controlled area, i.e. goes from airside to landside.

In the UK, in principle, there are restrictions on leaving the controlled area (imposed by HMRC) when there are international departures from that area, and therefore a process to be followed, hence why the term appeared in airport lingo.