When defining page types it's possible to specify which page types that should be available when creating a new page under that particular page type. This can help the editors a lot since it limits the choice down to the page types that are intended for that particular section of the website.
To specify the available types, set the AllowedTypes property of the PageType attribute. If AllowedTypes isn't set all pagetypes will be available.
If - for instance - you have a news list pagetype that you only want news pages to appear under you can specify it like:
[PageType("NewsList", "News list", "~/Templates/Pages/NewsListPage.aspx", AllowedTypes = new [] { typeof(NewsPage) })]
public class NewsListPage : CmsPage {
}