Although there is no upper limit, it is not endless. Even if all the actions of the highest group are linked together (which is actually impossible), a will look like 10 point if a certain bonus is still met.
Every project is different, and the A score is different. Now the highest A is on the uneven bars, 7.7 in international competition, and some people have the ability of 8 points.
B is the score of completion, and it is inferred from 10.
A+B is the total score.