Milestones are specific development goals within a project. Milestones can be assigned a priority (all priorities are defined in the admin configuration). Additionally milestones can be assigned to specific users within a project. Assignees are users that the milestone is considered to be "owned" by, or users that should be considered responsible for making sure the milestone is completed. Subscribers are considered to be more users that simply want to keep tabs on development or updates to a milestone.

Milestones are a powerful piece of the Project Manager as you can add a wide variety of items to a milestone as seen in the screenshot above.