(debian-policy.info)Priorities

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2.5 Priorities
==============

Each package must have a `priority' value, which is set in the metadata
for the Debian archive and is also included in the package’s control
files (see Note: Priority.).  This information is used to control
which packages are included in standard or minimal Debian installations.

Most Debian packages will have a priority of ‘optional’.  Priority
levels other than ‘optional’ are only used for packages that should be
included by default in a standard installation of Debian.

The priority of a package is determined solely by the functionality it
provides directly to the user.  The priority of a package should not be
increased merely because another higher-priority package depends on it;
instead, the tools used to construct Debian installations will correctly
handle package dependencies.  In particular, this means that C-like
libraries will almost never have a priority above ‘optional’, since they
do not provide functionality directly to users.  However, as an
exception, the maintainers of Debian installers may request an increase
of the priority of a package to resolve installation issues and ensure
that the correct set of packages is included in a standard or minimal
install.

The following `priority levels' are recognized by the Debian package
management tools.

‘required’

     Packages which are necessary for the proper functioning of the
     system (usually, this means that dpkg functionality depends on
     these packages).  Removing a ‘required’ package may cause your
     system to become totally broken and you may not even be able to use
     ‘dpkg’ to put things back, so only do so if you know what you are
     doing.

     Systems with only the ‘required’ packages installed have at least
     enough functionality for the sysadmin to boot the system and
     install more software.

‘important’

     Important programs, including those which one would expect to find
     on any Unix-like system.  If the expectation is that an experienced
     Unix person who found it missing would say “What on earth is going
     on, where is ‘foo’?”, it must be an ‘important’ package.  (1) Other
     packages without which the system will not run well or be usable
     must also have priority ‘important’.  This does `not' include
     Emacs, the X Window System, TeX or any other large applications.
     The ‘important’ packages are just a bare minimum of
     commonly-expected and necessary tools.

‘standard’

     These packages provide a reasonably small but not too limited
     character-mode system.  This is what will be installed by default
     if the user doesn’t select anything else.  It doesn’t include many
     large applications.

     No two packages that both have a priority of ‘standard’ or higher
     may conflict with each other.

‘optional’

     This is the default priority for the majority of the archive.
     Unless a package should be installed by default on standard Debian
     systems, it should have a priority of ‘optional’.  Packages with a
     priority of ‘optional’ may conflict with each other.

‘extra’

     `This priority is deprecated.'  Use the ‘optional’ priority
     instead.  This priority should be treated as equivalent to
     ‘optional’.

     The ‘extra’ priority was previously used for packages that
     conflicted with other packages and packages that were only likely
     to be useful to people with specialized requirements.  However,
     this distinction was somewhat arbitrary, not consistently followed,
     and not useful enough to warrant the maintenance effort.

   ---------- Footnotes ----------

   (1) This is an important criterion because we are trying to produce,
amongst other things, a free Unix.