Delete the search index before rebuilding?

Quick question. The 'Delete the index before rebuilding' option you have when rebuilding the search index, how is it affected by the 'Content type' option you can select above? For example, if I select the first content type of 'Direct messages' and then select to 'delete the index', does that ONLY delete the direct messages part of the index or the whole search index? It's not clear from the page.

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