See also
Originally, Sphinx was conceived for a single project, the documentation of the Python language. Shortly afterwards, it was made available for everyone as a documentation tool, but the documentation of Python modules remained deeply built in – the most fundamental directives, like function, were designed for Python objects. Since Sphinx has become somewhat popular, interest developed in using it for many different purposes: C/C++ projects, JavaScript, or even reStructuredText markup (like in this documentation).
While this was always possible, it is now much easier to easily support documentation of projects using different programming languages or even ones not supported by the main Sphinx distribution, by providing a domain for every such purpose.
A domain is a collection of markup (reStructuredText directives and roles) to describe and link to objects belonging together, e.g. elements of a programming language. Directive and role names in a domain have names like domain:name, e.g. py:function. Domains can also provide custom indices (like the Python Module Index).
Having domains means that there are no naming problems when one set of documentation wants to refer to e.g. C++ and Python classes. It also means that extensions that support the documentation of whole new languages are much easier to write.
To avoid having to writing the domain name all the time when you e.g. only describe Python objects, a default domain can be selected with either the config value primary_domain or this directive:
.. default-domain:: name
Select a new default domain. While the primary_domain selects a global default, this only has an effect within the same file.
The name of the default domain. Can also be None to disable a default domain. The default is ‘py’. Those objects in other domains (whether the domain name is given explicitly, or selected by a default-domain directive) will have the domain name explicitly prepended when named (e.g., when the default domain is C, Python functions will be named “Python function”, not just “function”).