Milestones allow you to mark key points within a project plan. They can be created from issues , /wiki/spaces/structure/pages/3667893958 or /wiki/spaces/structure/pages/3667894803 or flex items in the structure.
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Once a milestone is added to the chart, you can drag it to a desired date (if Manual Scheduling is enabled) and link tasks or groups to it. If a milestone has been manually scheduled, it will have a dark ridge down it's center:
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