With cross-domain tracking, you can track the goals and details of two different domains in a single experiment.
Let us consider that you have an e-commerce site with a cart/checkout section. The website and the shopping cart are located on 2 different domains, say, www.acme.com and www.acmecart.com. Here, if you want the user to browse the products from example.com and redirect to acmecart.com for the checkout process, then, it can be achieved through cross-domain.
NOTE: For Cross-domain functionality to work, please ensure that it is enabled.
To enable cross-domain, go to Admin Settings > CRO Settings > Cross-domain and turn on the cross-domain toggle 
How to use cross-domain?
Cross-domain uses third-party cookies to track the web page. Cross-domain functionality is available for Funnel Analysis, Form Analytics, A/B Testing and Split Testing.
Here’s how you can use it:
In the first step, enter the URL of the page on which the experiment should be created.
In the second step, enter the other URL. The domain of this URL can differ from the previous URL.
Enter the URL in the original and variation textbox. Once done, move to the next step.
Cross-domain can be used in tracking goals too. For eg: If you have ‘Track page visits’ goal and the URL points to a different domain or if you have ‘Track form submits’ goal and the action URL points to a different domain.
Enter the URL of the page for which form analytics to be done.
Load the form.
In the Define Track Conversion section, enter the URL of the page where the user should be redirected to. This can be a different domain from the experiment URL.
Run the experiment.
Enter the URL of the page for which the experiment should be created.
Once the experiment is created, make the necessary changes on the page and continue to goals.
Like split test goals, cross-domain can be used in tracking goals in AB test as well.
