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WBS is nothing but Work Breakdown Structure. It is mainly used in Project System (PS) module. Tcode: CJ20N. For any project defined we must have atleast one WBS. Within this WBS we can have networks. Networks are nothing but the series of activities. Material are used to perform the activities.
The WBS is the hierarchical breakdown of the project by deliverables. It is the actual structure we are used to seeing and using to organize the project. It allows us to breakdown the scope into more manageable pieces and provides a reporting structure that we use for planning, capturing performance and actuals.