Plugins may mixup any of the usages described above. There is no need to restrict to only one usage.
For example: A plugin for use as an authentication source might also be published as a web application, which allows the management of the user and group documents inside the plugin content store. Or: A design provider plugin may additionally register additional resources, like a new WebTML encoder which is used inside its design.
Another interesting example is the "OpenWGA Wiki Design" plugin. While it is a design provider for using OpenWGA web applications as Wikis it also is a web application itself, providing the Wiki online help. Both usage cases use the same Wiki design, but while the first may use any data entered by users the second will provide the contents of the plugin content store, which contains the help also in Wiki design.