DirectAdmin will always set your most recently created site as the default.
There are two reasons the default site may be important to you:
1) The default site can be found using the ~username instructions in your host's welcome email.
2) The default site can be found at public_html from your ftp login; the non-default site will require domains/example.com/public_html.
This post is being moved it's a User-Level request.
Jeff