The project Work Breakdown Structure (WBS) must contain all the work to be accomplished by the project.
PMBOK simply states:
“The WBS organizes and defines the total scope of the project…”
Thus, the 100% Rule states that your WBS must account for ALL of your project’s work. 100% of it.
The 100% Rule also means that every component in the WBS should be fully covered by the components below it.