Hi janfred, Some lines offer more guarantees than others. The lines all have different ways of regulating their inventory. On Carnival they're almost always available from the time they release bookings until the ship sails in multiple categories, they even have categories specially set up, typically at lower rates than assignable cabin Cats. IS is for Inside guarantee, OV for oceanview guar, BL for balcony guar and ST for suites. When they sell these categories out they open more in specific Cats that are normally assignable, 4A guar or some other.
NCL is well known for having their lowest priced categories in guar as well. With HAL the go to guarantee for promotions late, when the ships are getting full and they have scattered cabins in scattered categories, they'll lump them all together in the lowest category if inside or oceanview or whatever and offer the special rates only if you'll take a random assignment for the promotion.
Celebrity and RCCL and some others usually offer cabin assignments and don't often open guarantee offers. Their promotions are typically based on the cabin and category they still have open cabins in. When they sell those cabins they don't leave the Cat open and sell it as a guarantee often, they lower the price on the next higher category. You will sometimes see RCCL leave Cat Q inside open for guarantee but usually close to the sailing date when they're about out of cabins.
Princess often has a great promotion that is not based on a guarantee. They have been going at things different than the other lines lately. When they get low and have scattered cabins in various categories they open them all up and and sell everthing left at the same rate. So it becomes first come, first serve on whats left. That amounts to free upgrades inside a group of cabins. All inside cabins left for $___.__ or all oceanview at $___.__. These promos are great, you get the lowest price and can choose the best cabin, not gamble on what will be assigned.
Since they all do things different, there is really no way to know what promotions are affecting what sailings until you choose your ship and sailing date and make the call. At that point it's easy for your agent or the cruiseline to find the promotion that will best suit you and what you want.