Tower of God is comparable to the likes of One Piece when it comes to the scope and complexity of its worldbuilding. There aren't many series that this can be said about. There are countless key facets of the story and the world itself that fans have to remember to comprehend what's going on. That includes the different floors of the Tower, how characters process through said floors, the character rank system, etc.
With so much to remember and how long the story is (with no indication of ending anytime soon), fans can't be blamed for forgetting some things every now and then. That said, this is one of those stories where almost every detail means something. Thus, it's in every fan's best interest to be able to swiftly refresh their memory without having to reread everything.
Character Ranking System
There are two types of rankings in relation to characters in Tower of God. There's the Regulars ranking which depends on what floors a character has been able to pass. Then there's the ranking of Rankers which is reliant on the decision of the Ranking Administration Office. Regulars are ranked alphabetically from F to A, with F-rank being the lowest one can be and A-rank being the highest. As characters move up the floors of the Tower their rank increases.
Rankers are individuals who've ascended to the 134th floor of the Tower. The Ranking Administration Office decides whether a person is worthy of being a Ranker based on their power, influence, and recognition. Rankers are viewed as gods and are far higher than Regulars in the social hierarchy of the Tower. Rankers who are ranked within the top 10% of all Rankers are considered Advanced Rankers, while those within the top 1% are known as High Rankers.
Administrators are mysterious giant creatures that are in control of the Shinsu on each floor. They are responsible for administering tests to those who have a desire to climb the Tower. Rankers conduct the tests on behalf of the Administrators on the 134 floors that have been conquered by Jahad.
Special Characters
Throughout the Tower, there are several types of special characters and groups that are known for their influence and power. The Princesses of Jahad are a group of elite female warriors who are obligated to serve and defend Jahad's authority, in exchange for having received power from him. The Princesses are revered throughout the Tower and the position is highly coveted. They all refer to Jahad as their father but this is merely a formality as they are not biologically related to him.
Irregulars are anomalies within the Tower. They are people who were not selected to climb the Tower through the usual selection of Regulars. They entered the first floor from outside the Tower instead of from the middle or outer Towers. The fact that they entered the Tower on their own grants them certain benefits that others in the Tower don't share. This includes being able to use Shinsu without Guardian approval, as well as the possibility of killing Jahad. Those who did not enter the Tower on their own are unable to kill Jahad since it's against the laws of the Tower.
FUG and Wolhaiksong are two of the most well-known organizations that don't work for Jahad or the 10 Great Families. FUG is the most dangerous and mysterious criminal syndicate in the Tower. Their ultimate goal is to kill Jahad and the heads of the 10 Great families. Wolhaiksong's goal is to find a way outside the Tower.
The Floors
There are 134 conquered floors in the Tower. Each floor has at least one test that Regulars have to pass in order to move to the next floor. Regulars' ranks increase as they pass the tests on certain floors. For instance, they achieve E-rank after passing the 20th floor and D-rank after passing the 30th floor. Once they've achieved at least D-rank, they're free to get on the Hell Train. The Hell Train is an optional special test area for D-rank regulars on the 35th floor that allows them to climb up the Tower faster than if they used the ordinary test system.