Most reports indicate that official GTY assignments usually come fairly late (the barcode trick to find the assignment 45 days out doesn’t count) and by that time there are few if any cabins to move to.
Biker, who won Royal Up 41 days out.
With that flexibility comes the chances of having to wait. You are very unlikely to get into MTD before about 7 if you go into the no reservation line, though you should still book something (even if only 7:30 is available) and then adjust on board.
It's like just about any other TA. For round trips, it may not be the least costly. The usual advantage for A2S is one way flights in conjunction with a repo sailing.
The two scenarios reported are it’s either - GTY or <cabin number > after the booking number once barcode is decoded. Anything else I suppose is abnormal.