Although I give the clearance "direct destination" from time to time, I must confess I have no idea what it precisely means - working in upper area control, we sometimes get it from the approach unit and pass it on to the pilots; so far nobody has questioned it...and for me it makes no difference, as has been mentioned, the destination being normally so far away that it can be only 1-2 degrees difference.
I had expected the pilots to take it as "proceed direct IAF", which would also be the only thing that makes sense in case of coms failure.
Any approach controllers here who use it and can tell us what they expect?