Everything for managing the URL cache. HTML mode accumulates a list of the URLs you use in your HREFs SRCs etc., and saves the list across invocations of Alpha, so it can help you to put in new URLs rapidly.
Add selection - Adds the current selection to the URL cache.
Add clipboard - Adds the clipboard to the URL cache.
Clean Up - After a while the URL cache can get very large, and you may want to remove some URLs you do not need. You will be presented with a dialog box, as shown below, with a checkbox for each URL in your cache. Uncheck the ones you want to remove. Click "Uncheck all" to, guess what, uncheck all checkboxes. If there are more URLs in your cache than fit the window, there is a button "More" to take you to the next ones.
Clear - Removes all URLs from your cache.