new file mode 100644
@@ -0,0 +1,89 @@
+.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
+
+****************
+Variable Context
+****************
+
+Variables might only have an impact or can be used in certain contexts. Some
+should only be used in global files like ``.conf``, while others are intended only
+for local files like ``.bb``. This chapter aims to describe some important variable
+contexts.
+
+.. _ref-varcontext-configuration:
+
+BitBake's own configuration
+===========================
+
+Variables starting with ``BB_`` usually configure the behaviour of BitBake itself.
+For example, one could configure:
+
+- System resources, like disk space to be used (:term:`BB_DISKMON_DIRS`),
+ or the number of tasks to be run in parallel by BitBake (:term:`BB_NUMBER_THREADS`).
+
+- How the fetchers shall behave, e.g., :term:`BB_FETCH_PREMIRRORONLY` is used
+ by BitBake to determine if BitBake's fetcher shall search only
+ :term:`PREMIRRORS` for files.
+
+Those variables are usually configured globally.
+
+BitBake configuration
+=====================
+
+There are variables:
+
+- Like :term:`B` or :term:`T`, that are used to specify directories used by
+ BitBake during the build of a particular recipe. Those variables are
+ specified in ``bitbake.conf``. Some, like :term:`B`, are quite often
+ overwritten in recipes.
+
+- Starting with ``FAKEROOT``, to configure how the ``fakeroot`` command is
+ handled. Those are usually set by ``bitbake.conf`` and might get adapted in a
+ ``bbclass``.
+
+- Detailing where BitBake will store and fetch information from, for
+ data reuse between build runs like :term:`CACHE`, :term:`DL_DIR` or
+ :term:`PERSISTENT_DIR`. Those are usually global.
+
+
+Layers and files
+================
+
+Variables starting with ``LAYER`` configure how BitBake handles layers.
+Additionally, variables starting with ``BB`` configure how layers and files are
+handled. For example:
+
+- :term:`LAYERDEPENDS` is used to configure on which layers a given layer
+ depends.
+
+- The configured layers are contained in :term:`BBLAYERS` and files in
+ :term:`BBFILES`.
+
+Those variables are often used in the files ``layer.conf`` and ``bblayers.conf``.
+
+Recipes and packages
+====================
+
+Variables handling recipes and packages can be split into:
+
+- :term:`PN`, :term:`PV` or :term:`PF` for example, contain information about
+ the name or revision of a recipe or package. Usually, the default set in
+ ``bitbake.conf`` is used, but those are from time to time overwritten in
+ recipes.
+
+- :term:`SUMMARY`, :term:`DESCRIPTION`, :term:`LICENSE` or :term:`HOMEPAGE`
+ contain the expected information and should be set specifically for every
+ recipe.
+
+- In recipes, variables are also used to control build and runtime
+ dependencies between recipes/packages with other recipes/packages. The
+ most common should be: :term:`PROVIDES`, :term:`RPROVIDES`, :term:`DEPENDS`,
+ and :term:`RDEPENDS`.
+
+- There are further variables starting with ``SRC`` that specify the sources in
+ a recipe like :term:`SRC_URI` or :term:`SRCDATE`. Those are also usually set
+ in recipes.
+
+- Which version or provider of a recipe should be given preference when
+ multiple recipes would provide the same item, is controlled by variables
+ starting with ``PREFERRED_``. Those are normally set in the configuration
+ files of a ``MACHINE`` or ``DISTRO``.
@@ -13,6 +13,7 @@ BitBake User Manual
bitbake-user-manual/bitbake-user-manual-intro
bitbake-user-manual/bitbake-user-manual-execution
bitbake-user-manual/bitbake-user-manual-metadata
+ bitbake-user-manual/bitbake-user-manual-ref-variables-context
bitbake-user-manual/bitbake-user-manual-fetching
bitbake-user-manual/bitbake-user-manual-ref-variables
bitbake-user-manual/bitbake-user-manual-hello