This category is intended to collect all "devices" (often templates themselves) that are internal implementation details of Wikipedia templates.
These are not meant to be directly used by regular editors. They are sub-templates organized by a rootname of the primary template grouped in one place for the benefit of template implementors. This is, by the way, the reason why they are provided with very little external documentation.
To include a template in this category, add [[Category:Template implementation details]] to the <includeonly> section at the bottom of the template's /doc subpage if such a page exists. Otherwise, add <noinclude>[[Category:Template implementation details]]</noinclude> to the end of the template code, making sure it starts on the same line as the last character of the code.