WBS - Work Break Structure
Gantt Excel uses the standard hierarchical groupings for WBS. By convention, we use numbers and decimal points to indicate the level of the task.
Work Breakdown Structure (WBS) a hierarchical and incremental decomposition of the project into phases, deliverables and work packages.
Gantt Excel auto calculates WBS numbering for you based on task hierarchy so that you don’t have to do the hard work, you just need to figure out the major deliverables and the sub-deliverables and the tool magically takes care of it for you.
Everytime you add a task, delete a task, add a milestone or move a task/ milestone up and down, the WBS numbers update instantly.