HP PCL/PJL reference (PCL 5 Printer Language) - Technical Reference Manual Part II

17-26 An Introduction to HP-GL/2 Vector Graphics EN
Figure 17-10Relative Coordinates
Relative movement is useful in many applications where you know
the dimensions of the shape you want, but do not want to calculate
the absolute coordinates. For example, if you want a box 4 X-units by
8 Y-units, you can use the Edge Rectangle Relative (ER) command to
draw the box without having to calculate the absolute coordinates of
the opposite corner. (The ER command draws a rectangle using the
current pen location as one corner, and the specified relative
coordinates as the opposite corner.)
Absolute pen movement is the default mode; coordinates received
within a PU (Pen Up) or PD (Pen Down) command are interpreted
as absolute plotter units unless a PR (Plot Relative) command
establishes relative mode. As with absolute coordinates, the relative
units can be either user-units or plotter units, depending on whether
the SC command is in effect.
Note Relative increments add to the current pen location. The printer
automatically converts the new relative location to absolute
coordinates and updates the current pen location. Using relative
coordinates can be faster in cases where the I/O speed limits your
print speed, since relative coordinates are generally smaller
numbers and therefore transmit less data over the I/O.