This template should be placed at the end of a citation if the article incorporates text from that source that is not under copyright and there is no source specific template listed in Category:Attribution templates.

Example

change
...article text.<ref>An Old Book by A. N. Author {{PD-notice}}</ref>

This would display as

...article text.[1]

...and in the references section as

References

  1. An Old Book by A. N. Author This article incorporates text from this source, which is in the public domain.