This is a bit of a convenience function that comes in handy when you try out the plugin, or generally work with changing termbases a lot. When you add a glossary to a project, a termbase is created in the Glossaries folder. But when you remove it from a project, the files remain on disk. So if you add the same termbase again, a new one is created with a altered name (termbase.sdltb, termbase1.sdltb etc).
This can quickly clutter your Glossaries folder and make it difficult to remember which termbase is the one you actually still use. Clicking this button opens a dialog with a list of unused termbases: that is, termbases that are in the project's Glossaries folder, but not in the project itself. It comes with a big red intimidating cross, since gone is gone, and even though the termbase is not used in this project, you may still want to use it elsewhere. So, think before clicking OK!
Attention: due to a bug in Trados Studio, deleting will often fail. Once you open the project properties, all project termbases will be locked, and won;t be unlocked until you close Studio. A locked termbase can't be deleted, so the clean step will fail. You can use it after you restart Studio, and before you go to the project properties. That's unfortunate, because the sequence: remove termbase from project - cleanup, is the most likely use for the action, but you'll have to live with it for now.
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