Grouping several indexing configurations in one search category

You might find that you want to group the results from multiple indexing configurations in the same category. For instance, I have an indexing configuration for "typo3.org" but I want all search results to appear under the category "External URLs". This can be done by creating a special type of indexing configuration which only points to other indexing configurations:

This indexing configuration is not used during indexing but during searching. So a reconfiguration of the TypoScript to use uid 9 instead of 7,8 will yield this result:

TypoScript:

plugin.tx_indexedsearch.search.defaultFreeIndexUidList = 9,6,0