There are several methods for creating and modifying dependencies between tasks.
You can only create dependent relationships for items that are at the
lowest level of the Work Breakdown Structure (WBS). These are the lowest child-level, or lowest leaf nodes, of the WBS. They display under a parent level but have no WBS children of their own. For the purpose of task dependencies, these are referred to as "tasks" but they may be phases, tasks, labor codes, or lower indent levels, depending on the WBS structure.
When you select predecessors or successors for your dependent relationship, you can select a group of tasks (black bracket on Gantt chart). Because task dependencies must be at the lowest child-level, the dependencies are then created for the individual tasks within the group.