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

18-4 The Picture Frame EN
If a drawing does not meet the above criteria and the drawing is not
the same size as the picture frame, the HP-GL/2 plot size must be
specified to accomplish the desired scaling. If it is not specified, the
image is clipped to the effective window and no scaling occurs.
Note The above bulleted items are required for automatic scaling when the
picture frame size changes, without specifying the HP-GL/2 plot size.
However, if an HP-GL/2 plot size is specified, any unscaled HP-GL/2
image (any image created without the SC command) is automatically
enlarged or reduced to fit the PCL Picture Frame; the amount of
enlargement or reduction is determined by the picture frame scaling
factor (the ratio of the HP-GL/2 plot size to the PCL Picture Frame
size). See Chapter 19 to specify an HP-GL/2 plot size.