When you place something under a category, you can change how the category alphabetizes that article. This should be done when the title of the article is more than one word and several of the words are in the name of the category in question and many other pages in that category also have the name of the category in their title. In other words, rather than listing Article Q under A, you can have the category list it under Q.
To do this with code, you have to link to the category, then add a pipe after the category's name, then type what you want to list the article as. For example,
[[Category:Blog posts|Pages in categories]]
will cause this blog post to get listed as Pages in categories rather than Alphabetizing pages in categories.
To do it with Wikia's Add category, Click "Add category", press enter, then press the tiny button next to the name of the category with the two arrows going in different directions. Then type what you want the category to alphabetize the article as into the inputbox labeled "Alphabetize this article on the "Lists" category page under the name:". Then click "Save". Then save the article.
Most commonly, this is done with Lists, as the title usually contains "List of" and having 1000 pages under "L" in that category would be ridiculous. However, it is also recommended for other case where a large majority of the pages in a category start with the same word or words. Granted, it isn't that big of a deal, but it makes categories look much neater. Also, in certain cases where the name of the article is the name of the category or pretty close, list the article as single space so that it gets listed first.