In a WBS, the groupings of work are described in the project as individual elements in a hierarchy. The work is broken down in the project, step-by-step, until the level of detail is reached necessary to carry it out. These groupings of works are WBS Elements.
So, in GFEBS, a Work Breakdown Structure is a hierarchical arrangement of WBS elements under a project definition.
In GFEBS, there can only be one WBS directly below the Project Definition. This is known as the Level 1 WBS, and will have the same number (and usually name) as the Project Definition. They will also have the same name, for example, FY15 Reimbursables.
That leads us into the next item we need to discuss, the WBS Elements.