HP PCL/PJL reference (PCL 5 Color) - Technical Reference Manual

EN Raster Graphics 6-25
The command byte identifies two things: 1) the number of
replacement (delta) bytes that follow; and 2) where to position the
replacement byte string (the left offset). The replacement bytes are
some number (up to eight bytes) of consecutive bytes that are used to
create the new row from the seed row.
If more than eight replacement (delta) bytes are needed, additional
command byte/replacement bytes may be added, as shown below:
?*3m#W [(Command Byte)(1 to 8 Replacement
Bytes)][(Command Byte)(1 to 8 Replacement Bytes)]. . .
In the command byte, the upper three bits identify the number of
replacement (delta) bytes (which can be 1 to 8 bytes). The lower five
bits identify the location the replacement bytes are to be positioned.
This position is identified as the offset, or the number of bytes from
the treated byte. For example, if there are 5 replacement bytes and
the offset is 7, then the replacement bytes replace bytes 7, 8, 9, 10,
and 11 (the five bytes beginning at byte 7 from the seed row).
If there is more than one replacement in a row, the second offset is
counted from the next untreated byte in the row: the first byte
following the last replacement byte.
7 5 4 0
Number of bytes to replace (1-8) Relative offset from last untreated
byte