Guard
This page should correctly be named "[Guard]"; it is wrong due to technical restrictions.
Mission Control
The [Guard] section in rules(md).ini is one of the many 'mission' states a unit can be, wherein some basic behaviours can be defined or tweaked. Specifically when a unit is set to Guard, either via hotkey, buttons or automatically for units with DefaultToGuardArea set.
For a full list of available tags, see: Mission Control.
Infantry Sequences
Guard is also a tag within special 'sequences' in art(md).ini. In this case it defines the range of frames inside infantry's SHP art that should be displayed when set to Guard mode.
For a better understanding of function and available tags, see: Sequence.