HP PCL 5 Comparison Manual page 75

Default Galley Character (UI). Character code of the
character to be printed when a specified character is not
within any of the defined regions.
Number of Regions (UI). Number of regions for which
galley characters are defined. Regions are defined for a
table in which the first character code byte specifies the
row and the second byte specifies the column.
Region #x Upper Left Character Code (UI).
Character code defining upper left corner of Region #x.
Region #x Lower Right Character Code (UI).
Character code defining lower right corner of Region #x.
Region #x Galley Character (UI). Character code of
the character to be printed when a character within
Region #x is missing from the selected font.
If the value of the galley character field is 0xFFFF, then if
the font contains a missing character glyph, that glyph is
printed. The missing character glyph can be downloaded
using the PCL Download Character command with a
character code = 0xFFFF and a glyph ID = 0.
If both the character specified by the original character code
and by the galley character code are missing, the CAP is
advanced in accordance with previous PCL rules for
missing characters, that is, it is advanced according to the
current setting of HMI (Horizontal Motion Index).
The Galley Character Segment will be invalid if the format
number is not supported or if the segment size declared in
the Segment Size field is larger or smaller than required for
the number of regions (N). If the segment is invalid, the
font download will be ignored.
Galley Character Segments can be downloaded with any
Font Format 15 or 16 font, regardless of font type.
The Galley Character Segment can be used to implement a
requirement of the Microsoft Windows Version 3.1, Japanese
Version, Microsoft Standard Character Set Specification
(March 11, 1993), which states "when there is an output
request for a character of a specified typeface, even if the
glyph corresponding to the specified character code does not
Printer-Specific Differences 2-39