A summary WBS item is a main WBS item that contains one or more subitems. The summary WBS item together with its WBS subitems can be regarded as a single entity. Unlike WBS items, a summary WBS item is not a task. Its main function is to specify the time span in which all of its subitems must be completed. The moment you add a WBS subitem to a main WBS item, the main WBS item becomes a summary WBS item.

Summary WBS items can have dependencies on other WBS items and have a limited set of constraints. 'Start no earlier than' and 'finish no later than' are the only two constraints that can be applied to a summary WBS item.

The time span of the summary WBS item is determined by the earliest start date and the latest end date of its subitems. If the earliest start date or the latest end date of one of the WBS subitems changes, the start or end date of the summary WBS item is modified accordingly.

In this example, there are two WBS items, with a two-day dependency between them. Main WBS item 2 is the summary of subitems 2.1, 2.2 and 2.3. The duration of WBS item 2 is determined by the total time span of its subitems.