so I copied the blog app as starter template. I am trying to limit the categroy choice when submitting a new blog post to a custom namespace only. Right now the user has the choice to choose from any namespace. Where is the dialog in which I can change those settings? I cannot find it.
Categories uses TagInputField and you need to develop a small component by overlaying or looking  and adding a method filterFn to both occurrences of the SlingTreeLoader to display your namespace only.