The children need not be globally gifted, but may be gifted in one academic area, such as reading or math. Consequently, the children who are mathmatically gifted might be placed in one classroom while the verbally gifted are in another classroom. However, placing them in different classrooms is a problem if any of the children are globally gifted gifted since they can't be in both classrooms at the same time.
Movement in and out of these groups in relatively fluid. A child may be in the advanced group in math, but not in reading, and could be in the advanced group in math one year, but not the next year.
Cluster grouping is an inexpensive way for schools to meet the academic needs of gifted children. However, teachers must be able to differentiate instruction for the different levels of ability in the classroom.