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Milestones help track the progress of a goal over time. When creating a goal, you can add in as many milestones as you choose along with deadlines for when the milestone should be completed. By clicking the MORE DETAILS button next to a goal you can see the milestones for that goal and which ones have been completed so far.
Mark a check next to the milestone when completed and the progress bar will reflect accordingly. When you check a milestone off, a comment text box will automatically populate where you can describe any further details or accomplishments about the specific milestone.