HP 4 FAMILY - Manual (Page 92)

Vertical Underlining
In Japanese writing, vertical underlines are placed to the
right of vertical columns of text. This can be accomplished
in PCL 5 using the Fill Rectangular Area command.
An underline is simply a long thin black-filled box. The
length of the underline depends on the length of the text to
be underlined.
An example of vertical underlining is shown below. The C
program that follows was used to generate the PCL
commands for this example.
2-56 Printer-Specific Differences