A WBS Coding System:
The WBS chart below shows a WBS coding system. Notice how the codes let you identify a) the level of each element, and b) the parent-child relationships among WBS elements.

… and a little more terminology:
There is no set terminology for the elements at different levels of the WBS. Engineers tend to refer to systems, subsystems, and components; but others refer to tasks, subtasks, and activities. Again, the terminology differs from one user to another. In general, the elements may reflect products or activities, but they must together add to the total scope of work to be performed on the project. We suggest that you determine whether or not your organization (or the client) has a standard terminology, and then use that.

... However ...