To mark a task as a milestone, select Convert to milestone from task’s context menu.
This will add the text [milestone] to the task summary. Adding this text manually rather than via context menu, will also mark a task as a milestone, but will require a chart refresh.
For instance, see the issue STOR-209 that is marked as a milestone.
The duration of a task-milestone is always zero.
Milestone is never auto-scheduled by Gantt Suite, meaning milestone date is never changed by a linked tasks.
And vice versa, removing the text [milestone] from the task summary will convert milestone to regular task (after refresh). There is also Convert to task option in the milestone context menu. It doesn’t require chart refresh.