Timelines can display adornments, if the adornment has a $StartDate and $EndDate (or dates as per $TimelineStart and $TimelineEnd) These adornments are in fact shared with the map view of the same container, and use their colour, border, etc., values. The 'smart' elements of any such shared adornments (querying, etc.) only work in map view and so are ignored in timeline view. Adornments in maps of child/descendant containers are not shown. Thus a given map will always show all its adornments but the timeline of the same item will only show adornments with start/end dates.
Adornments are drawn from the top of the timeline view to the bottom of the bottom timeline band. Adornment staking order (if overlapping) is as per map view), though this can be still be adjusted in timeline view using the same controls as for map view.
Adornments draw in their map colour (i.e. $Color) and draw vertically across all bands on top of any band colour. The width of the adornment is controlled by its start/end dates as with notes.
Dragging a selected adornment right or left will move it and adjust $StartDate/$EndDate. Dragging the left of right drag-handle of a selected adornment allow separate adjustment of $StartDate/$EndDate.
As the Create Adornment dialog has no way to set $StartDate or $EndDate, it is advisable to create new timeline adornments in the map view of the timeline's container, even if said map view is not used for any other purpose.
Although the timeline view context menu allows you to add an adornment, it is better to create adornments in map view, then use their Info view to set $StartDate and $EndDate. Setting the latter will then make the adornment show in timeline view.