Multiple groups must be arranged in a hierarchy. For example, if your groups are geographical units, it is logical to arrange them according to size: first state, then city. To change the order, select a group break and click Move Up or Move Down.
When using multiple group levels, it is important to understand how the hierarchy affects the report output. When a break occurs at one level, it also forces breaks on variables with higher level qualifiers. For example, if state is defined as level 1 and city is defined as level 2, a break on state also means a break on city.