All formatters need to distinguish the text to be printed
from instructions about how to print; the latter is called
markup.  Procedural markup tells the software what to do
(space down, invoke a macro); generic markup describes the
thing to be printed (heading, cross-reference, etc.)  Troff
and TeX are examples of procedural markups; ODA and SGML
prescribe rules for generic languages; and good macro packages
for troff or TeX make them, more-or-less, generic markup
languages.

Many of the word processing packages (such as Word or Wordstar)
can only do procedural markup - the software is told by the
user "how to display it", rather than "what this is" - and leaving
the visual representation to preprogrammed rule sets (ie:
macros) - see GN5.