Hi AGES,
I may be wrong, as I have been so often in the past, but I believe it has something to do with changing frequency to another ATC unit.
One particular time I've heard this when I was working Birmingham Radar:
Me: G-ABCD, request frequency change to Wolverhampton, 123.XX
EGBB APP: QSY to Wolverhampton, Squawk 7000, G'day.
Hope that helps.
Everyone, please feel free to correct me