Brazilian jiu-jitsu is one of very few martial arts with a belt system that does not have a universal curriculum. There are a ton of schools that have curricula, and implement lesson plans with great detail. But if I said "In my school at blue belt you need to know sweep'X'" it is possible there are legitimate blue belts from lots of other schools that don't know that sweep. Regardless, being familiar with a technique that you cannot apply does not even mean much because it is application, not memorization that is primarily valued. So, it is not only possible that a blue belt might not be able to use the sweep, they might also not even play the type of guard where that sweep is mainly used.
A benefit of having an open technique set is flexibility. An open set allows students to emphasize techniques that their physical attributes help them excel at. Certain body types are more conducive to certain techniques. For example, a 6'3 250 lb. white belt will have more initial success pressure passing than mobility/agility passing. There are variations on every fundamental element of grappling. What matters most is that you make your variation functional. If your technique works against people bigger than you or higher ranked it is hard to say your variation of a move isn't right for you.
Creating your own style set requires instruction that addresses different styles so that each student is exposed to a style that might suit them best. I've spoken in the past about the benefits of being a generalist (someone who tries to become proficient in as many areas as possible), but there is no mandate. And you can achieve the rank of black belt with a very narrow set of techniques. This libertarian approach allows for an individual's game to truly be a personal expression of the art.
But since there is no universal curriculum, promotion through the system is 100% subject to the instructor’s personal interpretation of what makes a rank. If the instructor does not clearly communicate what they are looking for in a rank then the student has no guide. This can lead to confusion on what techniques to train, how to train them, and when to switch. Just look on-line and you can find article after article on "what you should be training at each rank." At white belt work on X, at blue belt work on Y, etc... Everyone is looking to gain a deeper understanding of how to navigate the hierarchy. But the truth is, since jiu-jitsu is so vast, there is no cookie cutter formula for all people at each rank. Internet aside, human uncertainty can mislead students into needlessly changing their focus. What should a student work on at each belt level? The cop-out answer... a little bit of everything. And often the difference in rank is not that you need a new technique, it's that you need to simply get better at what you are already doing.
At the core of discussing curriculum you are discussing what constitutes a specific rank. As an instructor, I want every student to know where they are on their path to the next rank, and what they have to do to get there. It is fair to say that more goes into a rank than just physical techniques. But in terms of technical ability, as I've said over and over again, jiu-jitsu places the focus on completing a function other than specifically "how." This can allow two people who are the same rank to have completely different techniques, tactics and even mindsets in accomplishing the same goal. This diversity of styles is not only one of jiu-jitsu's strengths but a core element of jiu-jitsu's identity.
photo credit // cauliphlower