HP 4 FAMILY - Manual (Page 81)

The following illustration shows how the PCL Typeface List
would look.
Vertical Rotation Segment
The Vertical Rotation Segment is used to define the lower
boundary of the rotation box used when the character text
path direction is set to vertical rotation. This is an optional
segment which may be downloaded with Font Format 16
TrueType fonts.
The following illustration shows an example of character
rotation. The boxes around each character represent the
vertical rotation box. The distance between the baseline
and the bottom of the character box is represented by the
Descender value in the Vertical Rotation Segment.
Printer-Specific Differences 2-45