HP LaserJet 4100 HP PCL/PJL reference (PCL 5 Printer Language) - Technical Ref - Page 28

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