The {{In category}} template is designed for use on the documentation subpages of templates. It produces a hatnote that specifies into which category the documented template is categorised.
Basic usage is along the lines of {{In category|Candidates for speedy deletion}}, which produces the following message:
It is possible to specify more than one category, e.g. {{In category|Candidates for speedy deletion|Candidates for speedy deletion by user}} produces:
Up to five categories are supported.
To override the "and" with another conjunction, use the |and= parameter.
For example, {{In category|Candidates for speedy deletion|Candidates for speedy deletion by user|and=or}} would produce:
This template does nothing to change the functionality of the documented template: in particular, it will not actually apply the categories. Its purpose is simply to produce a note about what the documented template does, but you will need to implement the template's actual functionality separately.