A milestone is generally considered to be a key event, typically a deliverable such as parts delivered. For progress meetings you may find it useful to filter your plan to display milestones only. Microsoft Project allows several ways of identifying milestones. For consistency, it is better to adopt one method only.

Method 1: On allocating a task with duration of zero, Microsoft Project will automatically display the task on the Gantt chart as a diamond symbol.

Method 2: You can identify tasks that have duration as also being milestones. To do this: Double-click on the Task Name to display the Task Information dialog box.
Select the Advanced tab, and then click in the checkbox to Mark task as milestone.

Marking a task as a Milestone

Both methods set the Milestone field to Yes. Inserting the Milestone field into a task table and setting the field manually, is another way to identify a task as a milestone.