What is the custom domain used for and where is it used? When I test the URL for the custom domain, I get an error message. Is this supposed to occur?
When I test the url this is the message i receive: Error 404
Page not found. If you want to change the content of this page, go to your account Settings / Root domain.
I assume we are talking about custom "tracking" domain right? What tracker are you using?
Some hosted trackers offered or still offer shared domains, that any user of their trackers could use. However, this caused some trouble in the past, when this shared domain got flagged, everyone was affected. So these days, it's highly recommended to use a custom domain for your tracker, that's only yours and nobody else can use it.
This domain is then part of your campaign links, that you use at traffic sources to setup your campaigns there.
Did you read anything about tracking at all, I don't want to go into the very basics in this thread, unless really needed.