The Box is defined by the position of its corner in Physical Coordinates in the Plot Page by command
SET BOX_LOCATION Xmin Xmax Ymin Ymax
The default Box for the default Plot Page can be accessed by typing
SET BOX LANDSCAPE or SET BOX PORTRAIT as required. This
default box is however usually inadequate for maps, where the User
Coordinates in X and Y may be related. The box must of course stay
within the limits of the PLOT_PAGE, so that attempts to define a
PORTRAIT page in a LANDSCAPE page (or vice versa) will fail: an
apropriate SET PLOT_PAGE should have been issued before the SET BOX
command.