Suposse you have a blogpost blog like I do ;-) . Now suposse that someone wants to add a certain category of your blog into a planet. You could follow this instructions and that's all, you have your own feed for a special category.
But it happens that I blog about Debian. In english and in spanish. And there are two planets. So, I asked myself if I could filter by two categories (and perhaps, more). It's possible:
So, for example, my spanish Debian posts would be:
(If the feed above fails, you are probably having a problem with the ñ in it. Try copying and pasting the URL feed).
And, in english:
Of course, if you happen to be interested in my KDE posts, just change debian with kde in the above URLs ;-)