The USAD is unique in terms of the breadth of knowledge and diversity of teams that it requires.
A team from a school can have up to nine members on it. Three members are called honors and their GPA can be anything up to and including 4.0 (Note that GPA is determined in a unique way for Academic Decathlon by ignoring honors classes and most electives). Three members are called scholastic and their GPA can be anything up to and including 3.74. Three members are called varsity and their GPA can be anything up to and including 2.99. Therefore, teams are forced to find people with below a "B" grade average to host a full team.
It is not uncommon for students to compete in a higher category. For instance, a student with a GPA of 2.8 would normally compete in the varsity category, but could compete in scholastic or honors. Generally it is to the students advantage to complete in the lowest category they can, as the scores in varsity are typically lower than those in scholastic, and those in scholastic are typically lower than those in honors.