From Protege Wiki
This category is a subcategory of the Plugin category. A "slot widget plug-in" is a particular type of Protege plug-in that allows developers to extend the group of components that support slot and property value entry. Documentation on how to write a slot widget 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.