That is not exact. There are two types of fares where
DCL assigns your stateroom : GTY and *GT.
The first one is a non-restricted/non-discounted fare released when a majority of staterooms in a specific category are sold-out. You are guaranteed that specific category or higher. For example, if you book a 4A verandah GTY, you can only get that category or a Concierge stateroom. They never offer the possibility of booking a specific stateroom while that specific category is made available for a GTY booking. Those bookings follow the exact same fare rules as a "regular booking", so they can be transferred or cancelled under the general rules. A category can go to GTY bookings at any time, and often does on the first day bookings are made available on very popular cruises.
A *GT booking is also called a restricted booking. Those fares are discounted and used to fill the ships. They are usually offered after the PIF date, but sometimes before if a specific cruise is not selling to DCL's satisfaction. You can only pick a specific type of stateroom : Inside (IGT), Outside (OGT) or Verandah (VGT). You can therefore get the lowest possible category of the type of room you booked, or anything higher. Those bookings are non-refundable and non-transferable, and must be paid in full at the time of booking, unlike a GTY cruise.