You have almost certainly seen that in your cabin reservation, you have the possibility to mark if you want a guaranteed cabin, You can find it with the acronym GTY in English, this is that they give you a cabin, in the modality you have chosen, but You still do not have one in particular, you will know it shortly before setting sail. The average is approximately 48 hours before sailing, although Royal Caribean usually assigns it 10 days before.
With this modality you find very good prices, But each shipping company has a different policy on this, and sometimes more than the price they reward your uncertainty assigning you a cabin of more category, simply because they sold all those that were already assigned. Be clear that they will never give you a lower category one.
The great uncertainty is that you do not know when the company is going to assign you a cabin, so if you are very clear about where you want to travel on the ship, choose directly the one that best suits you when making your reservation. On this article you have some tips to choose the best cabin.
An advantage (I see it that way) that this system has to choose from, is that if you have a guaranteed cabin, you will discover new parts of the ship, that perhaps by yourself they would never have been your option.
It is true that not all shipping companies have this possibility of guaranteed cabin, and also what happens is that they take it out in low season, but not in high season. Be that as it may, always find out if you have this type of rate. Shipping companies have this option because they prefer to sell the higher-end cabins even at a lower price, before they are empty.
If you are going to travel as a family, or in a group and you want to be all together, this guaranteed cabin option is not worth it, because you will not be able to choose adjoining cabins.
I hope I have solved your doubts and that you have more criteria for