This template is intended to allow novice template writers an easy way to write a template which can be transcluded for demonstration purposes, but in a way that avoids adding the categories to the demonstration page.
For instance:
{{ User:JackSchmidt/TL AddCats | Articles needing attention | Even-toed ungulates | 1973 in sports | display=on }}
Indicates the desire to add three categories to a page, but this desire is suppressed since the template is being transcluded for display purposes only:
 | This template would include User:JackSchmidt/TL AddCats/doc in the following categories: Articles needing attention , Even-toed ungulates , 1973 in sports |
To use this template in another template, use the syntax
{{ User:JackSchmidt/TL AddCats | Articles needing attention | Even-toed ungulates | 1973 in sports | display={{{display}}} }}
and then your template will behave normally (adding categories to the transcluding page) if your template is not called with the display=on parameter, but will go into "display only" mode if called with a display parameter set to on (or yes or true).