Borrowed from Doug's doc
"Datums should be labeled (-A-, -B-, etc.) when selected.
No drawing will reuse a datum letter once used so this is a safe practice.
Need to filter the ‘History’ list appropriately when selecting datums
Primary = Plane Secondary = Plane, Line, Cylinder axis etc.
Tertiary = Plane, Line or line feature, Point
May be a good idea to label features as datums when creating them."