With the help of your custom track domain, it is possible to do the tracking in a dedicated domain. This allows tracking data to be collected and processed even if your website visitors are using browser extensions to prevent a page from communicating with analytics tools, such as ad blockers. The use of the custom track domain thus increases the quality of your data.
You can find the latest documentation at this link: Custom Track Domain.
There you can also download the documentation as a PDF file.