This question was answered on our Cruise IQ show on November 1, 2017.
Booking a guarantee stateroom means that at time you make your reservation, you will select a category type (interior, ocean view, balcony or suite), but a specific stateroom number and location is not assigned. Depending on the cruise line and method of booking, you may be able to opt for a guarantee directly or by specifically requesting one.
You may be wondering, why would I want to do this? The simple answer is that guarantees present opportunities for upgrades to higher categories. Within each main category type, there are a number of sub-categories, each with distinct attributes. For example, an interior guarantee could allow you to pay the lowest available price for an interior room (usually located on the lowest deck or a less desirable location) and receive a room on a higher deck for the same price. It is even possible, in some circumstances to go from one main category to another, like interior to ocean view or even balcony.
There are some factors to consider in determining if booking a guarantee is right for you as well as helping you manage your expectations. Firstly, getting an upgrade of even one category is not always a sure thing. In fact, you may end up in the lowest sub category. When you choose a guarantee, you can literally be assigned anywhere, which could be the best or worst room on the ship. In our experience, cruise lines do try to assign the best available rooms to guarantees. Past guest status as well as general availability of rooms on a cruise can affect the assignment of a guarantee. A person who has cruised several times with a cruise line, may get a better room assigned and if the ship has a lot of empty rooms, the chances of an upgrade are generally increased.
Those that require a specific room for mobility reasons or just want a specific location should never book a guarantee. Also, if special room attributes like having a sofa-bed versus upper berths are important, then take a room assignment. Assignment of a guarantee room can occur anywhere between the date of booking and the date of departure. While you generally need to accept whatever room you are eventually assigned, most cruise lines will allow you to switch between rooms in the same sub-category or upgrade to a higher category for an additional cost.