Classes in bands are determined by skill level. Regional bands are geared towards bands just starting out. It allows schools with lower budgets to still perform, without the higher competition, skill set, and budget of A and Open class bands. US Bands classifies these bands as entry-level.
A class bands are the standard for performance. A class bands show fundamental through intermediate skills. The majority of bands fall into this category.
Open class bands are the most advanced groups. Open class bands have higher budgets, and perform with greater skill.
Band size does factor into class. A band in I Open is the same size as II Regional A or II A.