The fundamental operation of the GNU troff formatter is the
translation of the groff input language into a device-independent
form primarily concerned with what has to be written or drawn at
specific positions on the output device. This language is simple and
imperative. In the following discussion, the term command always
refers to this intermediate output language, and never to the
groff language intended for direct use by document authors.
Intermediate output commands comprise several categories: glyph output;
font, color, and text size selection; motion of the printing position;
page advancement; drawing of geometric objects; and device control
commands, a catch-all for operations not easily classified as any of the
foregoing, such as directives to start and stop output, identify the
intended output device, or place URL hyperlinks in supported output
formats.