A milestone is a reference point to mark major events in a project and is used to monitor a project's progress.

It is possible to mark any WBS item with any duration as a milestone. You can set different milestones at different phases of a project to observe as the project enters or completes these phases.

At the Project items selection level, use the Milestone? field to indicate if a WBS item is a milestone. A WBS item is considered as a Milestone if its Effort is set to zero hours.