As explained in the activity type section, an activity can be an appointment.
These are typically used for milestones, or for scheduled important meetings during the project.
This page will explain how this can be done directly from the timeline.
Calendar appointments can be added to a project without being part of the timeline as explained here.
Timeline calendar planning
Planning from the timeline is clicking on the calendar icon in the activity type row.
This will open the appointment creation page explained here.
Edit or delete planning for the timeline
Editing or deleting can be done by simply opening the appointment and edit.
Deleting is done by removing the appointment from the calendar.
Effects on moving the timeline
If the activities are moved either manually or as a result of their dependencies, the appointments will be changed as well.
The users and the start time and end time of the appointments aren't changed. Only the dates.