From Protege Wiki
This category is a subcategory of the Plugin category. A "project plug-in" is a particular type of Protege plug-in that allows developers to manipulate Protege's user interface and project files during various points in a Protege project's life-cycle. Documentation on how to write a project plug-in is available on the Protege website. Please note that this plug-in type is specific to the Protege 3.x (and earlier) series.