Example:
Create an initial user coordinate system to zoom in
the values: 1 unit to 4 pixels (4 px/unit or 4 px/mm).
If 400px/4 = 100 unit, the default user pixel densty is 4px/unit.
If 1 unit = 1 mm, initial density is 4 px/mm
This font size is
10 (read: 10 mm).
width = height = 10 (read: 10 mm).
width = height = 1 (read: 1 mm)