One diagram (in black, below) sorts out NCT's system into 3 categories. First, the diagram divides NCT into its city units - the Seoul team, whose name was revealed to be NCT 127; the Tokyo team; and the Beijing team. Each of the city units will debut with a fixed number of members, and will focus on promoting in their respective cities/countries. More members can be added to each of the city units, but fixed members cannot be subtracted from them.
NCT U is a unit which objectively focuses on the "unit" aspect of NCT, and freely enlists members from all three of the city units for individual promotions. NCT U is a unit in which any NCT member can be freely added to or subtracted from, without any restrictions.
About the relationship between S.M.ROOKIES and NCT, fans suspected that all male trainees who are currently a part of S.M.ROOKIES will eventually become a part of NCT.
In another diagram (in yellow, below), fans compare NCT to a school system. The city units are like classes, with a fixed number of debut members per class. NCT U gets compared to extracurricular activities, while NCT Dream is a unit that consists of the equivalent of 1st years at a school. Those in older years cannot join NCT Dream.
Of course, many fans still remain puzzled about NCT, since no one can know for certain how many more units can be formed. Did these diagrams help you understand NCT better, or nah?