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In page Work breakdown structure:

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It is common for work breakdown structure elements to be numbered sequentially to reveal the hierarchical structure. The purpose of the numbering is to provide a consistent approach to identifying and managing the WBS across like systems regardless of vendor or service.[1] For example, 1.1.2 Propulsion (in the example below) identifies this item as a Level 3 WBS element, since there are three numbers separated by two decimal points. A coding scheme also helps WBS elements to be recognized in any written context, such as progress tracking, scheduling, or billing, and allows for mapping to the WBS Dictionary.[citation needed] It is a preferred practice that the Statement of work or other contract descriptive include the same section terms and hierarchical structure as the WBS.