The explanation of Guarantee Status at the first post in this thread is not necessarily correct. I booked and got a Guarantee Status for a 5E stateroom. These are all at the stern of the ship, which is what we wanted. When they assigned a room, Disney put us in a Cat 7 mid-ships by the elevator and refused to accept my complaint that their assignment was a much worse room.
Essentially they said that Guarantee Status for 5E means 5E or higher category to them. We had even asked the cast member who booked us originally to mark the record that we did not want an upgrade except to concierge. This request was ignored.
Beware of Guarantee Status.
Cap
When almost all cabins of a specific category are sold, DCL will often change the cabin selection to GTY. Which means they choose the room, and you will receive at least the category you booked, or possibly a more expensive one. ("Better" is subjective.) People who book a category as GTY don't get any discount for booking this way. They are paying the same, or often more, than their neighbors who booked early and were able to choose a specific cabin. DCL has operated this way at least since I first looked into them in 2005. Cabins booked this way have the same deposit & cancelation policy as those who choose their own cabin.
This thread describes a more recent method for DCL to fill empty cabins relatively last minute. In the cruising world, 3 months is last minute given that most sail dates are released 2 yrs in advance. Yes, I've booked 2 yrs out many times & cruises have sold out on the first day of release, 2 yrs in advance , back when there were only 2 ships. But I digress....
So to sell empty cabins they came up with 3 choices for discounted pricing: IGT, OGT, VGT. These are usually offered after the final payment due date and have very strict rules regarding booking:
No name or date changes.
No cabin changes.
No refunds.
Full payment due at time of booking.
Not combinable with onboard booking or other discounts.
Probably some others I can't think of at the moment.
Those booking IGT are guaranteed at least an inside cabin or better.
OGT, are guaranteed oceanview (window) or better.
VGT are guaranteed a verandah.
So looking at the classic ships, if one doesn't want decks 1 or 2, it would be best to book VGT since there are no verandah cabins on those decks.
If you just want to be on the ship and don't care where, IGT will save more money.
In 14 Disney cruises, we've always chosen our cabins and once we were still upgraded (from a 5E to a 4C) on the Dream so we were very happy.
The only way to get a certain cabin with no chance of being moved is to book a date & category that still allows you to choose your cabin. Have DCL or your TA make a notation on your reservation to not move you. When you book GTY they can & will move people around as needed.
hth