docs: document how spack picks a version / variant (#41070)

lib/spack/docs/build_settings.rst

@@ -37,7 +37,11 @@ to enable reuse for a single installation, and you can use:
    spack install --fresh <spec>
 
 to do a fresh install if ``reuse`` is enabled by default.
-``reuse: true`` is the default.
+``reuse: dependencies`` is the default.
+
+.. seealso::
+
+   FAQ: :ref:`Why does Spack pick particular versions and variants? <faq-concretizer-precedence>`
 
 ------------------------------------------
 Selection of the target microarchitectures

lib/spack/docs/frequently_asked_questions.rst

@@ -0,0 +1,77 @@
+.. Copyright 2013-2023 Lawrence Livermore National Security, LLC and other
+   Spack Project Developers. See the top-level COPYRIGHT file for details.
+
+   SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+==========================
+Frequently Asked Questions
+==========================
+
+This page contains answers to frequently asked questions about Spack.
+If you have questions that are not answered here, feel free to ask on
+`Slack <https://slack.spack.io>`_ or `GitHub Discussions
+<https://github.com/spack/spack/discussions>`_. If you've learned the
+answer to a question that you think should be here, please consider
+contributing to this page.
+
+.. _faq-concretizer-precedence:
+
+-----------------------------------------------------
+Why does Spack pick particular versions and variants?
+-----------------------------------------------------
+
+This question comes up in a variety of forms:
+
+ 1. Why does Spack seem to ignore my package preferences from ``packages.yaml`` config?
+ 2. Why does Spack toggle a variant instead of using the default from the ``package.py`` file?
+
+The short answer is that Spack always picks an optimal configuration
+based on a complex set of criteria\ [#f1]_. These criteria are more nuanced
+than always choosing the latest versions or default variants.
+
+.. note::
+
+    As a rule of thumb: requirements + constraints > reuse > preferences > defaults.
+
+The following set of criteria (from lowest to highest precedence) explain
+common cases where concretization output may seem surprising at first.
+
+1. :ref:`Package preferences <package-preferences>` configured in ``packages.yaml``
+   override variant defaults from ``package.py`` files, and influence the optimal
+   ordering of versions. Preferences are specified as follows:
+
+   .. code-block:: yaml
+
+      packages:
+        foo:
+          version: [1.0, 1.1]
+          variants: ~mpi
+
+2. :ref:`Reuse concretization <concretizer-options>` configured in ``concretizer.yaml``
+   overrides preferences, since it's typically faster to reuse an existing spec than to
+   build a preferred one from sources. When build caches are enabled, specs may be reused
+   from a remote location too. Reuse concretization is configured as follows:
+
+   .. code-block:: yaml
+
+      concretizer:
+        reuse: dependencies  # other options are 'true' and 'false'
+
+3. :ref:`Package requirements <package-requirements>` configured in ``packages.yaml``,
+   and constraints from the command line as well as ``package.py`` files override all
+   of the above. Requirements are specified as follows:
+
+   .. code-block:: yaml
+
+      packages:
+        foo:
+          require:
+          - "@1.2: +mpi"
+
+Requirements and constraints restrict the set of possible solutions, while reuse
+behavior and preferences influence what an optimal solution looks like.
+
+
+.. rubric:: Footnotes
+
+.. [#f1] The exact list of criteria can be retrieved with the ``spack solve`` command

lib/spack/docs/index.rst

@@ -55,6 +55,7 @@ or refer to the full manual below.
    getting_started
    basic_usage
    replace_conda_homebrew
+   frequently_asked_questions
 
 .. toctree::
    :maxdepth: 2

lib/spack/docs/packages_yaml.rst

@@ -256,6 +256,11 @@ on the command line, because it can specify constraints on packages
 is not possible to specify constraints on dependencies while also keeping
 those dependencies optional.
 
+.. seealso::
+
+   FAQ: :ref:`Why does Spack pick particular versions and variants? <faq-concretizer-precedence>`
+
+
 ^^^^^^^^^^^^^^^^^^^
 Requirements syntax
 ^^^^^^^^^^^^^^^^^^^
@@ -435,6 +440,11 @@ them if it must, due to other constraints, and also prefers reusing
 installed packages over building new ones that are a better match for
 preferences.
 
+.. seealso::
+
+   FAQ: :ref:`Why does Spack pick particular versions and variants? <faq-concretizer-precedence>`
+
+
 Most package preferences (``compilers``, ``target`` and ``providers``)
 can only be set globally under the ``all`` section of ``packages.yaml``: