In level 9 and 10 in ALL regions it is based on a score. This is regulated by USAG and it used to be set at 34 for all regions, but 2 years ago USAG determined that each region can set their own qualifying score to Regionals for L9 and L10, but it is a score, not placement or percentage. Region 8 then raised their score to 35 due to the high number of L9 and L10 competitors. The reason it is a flat score and not a percentage or placement is because at L9 and L10 Regional is the qualifying meet to go to Nationals or Eastern/Westerns. So, at Regionals it is the top # of girls in each age group that advance.
For Levels 8 and below, each region determines if they will have a regional championship and how it will be structured, so this varies greatly between regions. Region 8 has 'state team' of top 8 from each state for each level and then additional slots are for "All Stars." R8 figures number of All stars by deciding first how many sessions of each level they will have at regionals (Which by the way they have decided to add 2 L6 sessions this year according to meeting minutes recently posted on R8 website). Once they know how many sessions they will offer, they will then know how many competitors they can fit into each session for a total number of All star slots. These slots are then divided up among the 8 states in the region based on how many state qualifiers there are from each state so that each state gets the same percentage of all star slots relative to their total number of competitors. The specific percentage will vary slightly from year to year based on total number of competitors changing.