In these examples, milestones are represented by the icon, and fixed milestones are represented by the icon.
Milestones that have not been fixed are scheduled on the date initially filled in on the line. If a milestone has predecessors or successors, the milestone is scheduled in the same way as a task. The difference is that a milestone does not have a lead time, and is therefore always scheduled on one single day.
If a milestone is a successor of a project component, it will follow the project component, as you can see from the first milestone in the example below. If the milestone should end at the same time as the project component, it is scheduled on the finish date of the project component, as you can see from the second milestone:
If you fix milestones to a date, they will remain on that date:
In practice, such a milestone will usually not be linked to particular project components and will not have to start or end on the same date, for instance, but even if that is the case, your fixed milestone will remain where it is: