Search Index - not all areas available to exclude

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I am using the concrete5 search block, which works well for my content.

In system and settings I selected the "Search Index" option to exlude content I don't want included in the search using the Blacklist option. Unfortunately, not all the areas in my site that I wish to exclude show up in the Areas list. For example I have Page Footer but no Page Header.

Is there a way I can add to the items displayed in the areas section? I would really like to exclude the site navigation from the search. Does anyone know how concrete5 determines the areas to display?