HP 5961-0509 manual Coordinates as the opposite corner, SC command is in effect

	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.

17-26An Introduction to HP-GL/2 Vector Graphics EN