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Top page is the first page, i.e. the page, which is loaded when hitting www.yoursite.com - which opens the index.php in the XOOPS root folder.
If you have not specified a module as the start page (in System Admin -> Preferences -> General Preferences) you'll just have a page without other contents than the blocks, you specify to be visible on the top/all page(s)
If you want the block only to show on the top page and e.g. not in the forums or articles or whichever modules you have installed, you should only enable it for the top page. If, however, you want this block to be visible in other modules' pages, choose those modules or "All pages" in the multiple select box.
To make the block visible to anonymous (non-logged-in) users, you'll also need to go to System Admin -> Groups and edit the anonymous users' access to include this custom block.
