Milestone tasks are very similar to regular tasks on a project with one major exception. They are only used for establishing a milestone on a project. Once set up, the Milestone task is locked and cannot be changed back to a regular task.
Milestone tasks have no resources or budget hours associated with them.
The three required fields are Milestone Type, Billing Amount, and End Date. All of these can be changed anytime.
Billing Amount and End Date are not calculated, and therefore remained unlocked. They can be adjusted as project needs change.