"Unused()" is a helper procedure written by the "fathers" of TACharts in order to silence the compiler if a parameter passed to a procedure is not used. Suppose you write a handler to a button OnClick event, it is supposed to show a message:
procedure TForm1.Button1Click(Sender. TObject);
begin
ShowMessage('Button1 was clicked');
end;
This works fine, but when you look at the message window after compilation you will see a message saying that parameter Ssnder is not used by the procedure Button1Click. This is a hint that the parameter could have a real, important meaning and you just forgot to call it. Of course, this is not true here, and the hint is a false alarm. In order to keep the message window free for "real" hints and warnings you can call the procedure Unused with the parameter Sender. This procedure does not do anything, it just has the parameter in its parameter list, and therefor the parameter is no longer "unused" by the calling procedure Button1Click.
procedure TForm1.Button1Click(Sender. TObject);
begin
Unused(Sender);
ShowMessage('Button1 was clicked');
end;
Another method would have been to right-click on the hint in the message window and select "Hide message by inserting IDE directive {%H]". I don't know why the inventors of TAChart did not choose this "official" way, maybe this directive did not exist in those days.