Project tasks represent individual actions within a set of actions that must be completed to achieve a goal while project milestones are used to mark a point in your project, usually completion of a set of tasks or as a project health check to determine if you’re on schedule.
A milestone can be converted to a project task by adding a positive number of estimated work or tracking time against the milestone. You can edit a milestone and assign a resource with 0 hours of estimated work without converting the milestone to a project task. However, if the resource tracks time against the milestone it will be converted to a project task.
Conversely, a Project task can also be converted to a milestone by removing positive estimated work.