Visual Representation of a plugin in a use case diagram built with starUML

The question says it all, how can I build a visual representation of a plugin? I have the idea I should regard it as something else or just not display but I can't find any evidence (in high enough amount) to be sure of this.

Should I not display a plugin in my use case in any situation at all?

Do I need to represent a plugin as a package or actor instead? (If so what connection should I have, Include?)

Or should I just represent it as I would an interface?

Maybe I'm just off track here, last time I made a UCD was a year ago or something, it's a开发者_运维知识库ll slipping away when you don't use stuff! So I wouldn't mind some "beginners" advice here :)

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Use cases are for analysis, not design, therefore they should omit architecture structure. When you have a plugin, it is a system and therefore might be the system under development or an actor. If it is the system under development and use cases consider only this plugin, you show it using the boundary box in the diagram encompassing the use cases (some tools don't allow that, they make it implicit). Otherwise, if you have use cases which describe behaviour of the system, which interacts with the plugin, you depict the plugin as an actor.