Basically they have to make a least the USAG qualifiying score and usually the want them making above 34...closer to 35 or 36. They may move but may not compete if they do not have all their skills. It is really an indiviual decision at our gym. The coach may move an older gymnasts where he would hold back a younger one if they were weak. We had a level 7 who did not compete the first 3 meets because she did not have her flyaway. Technically she was level 7 but was not allowed to compete until she had all her skills consistnely. Usually there is much angst about these decisions.
The gym I am at does not look at scores to determine who moves up. Some girls that score 34.00s move up, while there are others in the 36s that may not. It is all based on whether the gymnast can get her skills for the next level and whether or not she is mentally ready to compete at that level.