Milestones allow you to represent activities that do not require progress percentages. They are generally activities that are either finished or not. Therefore, Milestones only have only three statuses: Not Started, Completed and Signed Off.
Milestones do not contribute to the Project Completion Percent.
To create a Milestone, click on the Hammer Icon in the Tool Bar.
You can edit a task to become a Milestone.
Currently, users can still report progress as a percentage on milestones using the Mobile App. For example, a user can report 40% on a milestone from the Mobile App, and the status will be In Progress. This will force the Milestone into the In Progress status. This status will show up on the Desktop version, but wont include the percentage. On the Desktop, you are still only allowed to complete a milestone, not add a percentage. This will be updated in the near future.