Cross domain tracking allows a website to maintain a visitor's session and original traffic source even when that visitor navigates to a different domain, such as a checkout system on a separate domain. Without cross domain tracking, the visitor would appear as two separate visits and the original traffic source would be lost. The document discusses how Google Analytics uses first-party cookies to track visitors within a single domain but not across domains, and explains two methods for setting up cross domain tracking using the autoLink plugin and referral exclusion lists.

1. UNDERSTANDING CROSS DOMAIN
TRACKING
INTERNAL: Google Confidential and Proprietary

2. Understanding Cross Domain Tracking

3. Understanding Cross Domain Tracking
A Visitor comes originally from traffic source eDM to your website www.abc.com. Same visitor
navigates towards checkout on your website, but your checkout system is on a different
domain abccart.com.
Without cross tracking original visitor session will be lost when your visitor crosses over to your
checkout domain, therefore your visitor is split into 2; Visitor A and Visitor B even though it’s
the same visitor. Most importantly, the original traffic source eDM is lost, now traffic source for
visitor B will be abc.com (self referral)

4. Understanding Cross Domain Tracking
The analytics.js library uses a single first-party cookie to store the Client ID
that can only be accessed by the domain on which it is set. If your visitors
traverse from your domain to a third-party domain (e.g. payment gateway) this
cookie is lost therefore original session is lost. Cross domain tracking ensures
that the analytics.js cookie data is maintained across each of these domains
traversed.

5. Understanding Cross Domain Tracking
2 Simple Setups
Setting Up Cross Domain Tracking
Update Universal Analytics Code – autoLink Plugin
Setup Referral Exclusion List

6. Understanding Cross Domain Tracking
Setting Up Cross Domain Tracking
autoLink Plugin

7. Understanding Cross Domain Tracking
Loads the linker plugin
Define which domains to
auto link e.g. shopping cart
domain
Setting Up Cross Domain Tracking
autoLink Plugin

8. Understanding Cross Domain Tracking
Setting Up Cross Domain Tracking
autoLink Plugin
If you have implemented the autoLink plugin correctly,
whenever you click on links that lead to the 3rd party (e.g.
shopping cart) domain, you should see the URL is attached
with the _ga query string as follows:

9. Understanding Cross Domain Tracking
Setting Up Cross Domain Tracking
autoLink Plugin
Now on the third party domain, update the create function to
enable linking by setting the allowLinker attribute to true
ga('create', 'UA-XXXXXX-X', {
'allowLinker': true
});

10. Understanding Cross Domain Tracking
Setting Up Cross Domain Tracking
Referral Exclusion List

11. Understanding Cross Domain Tracking
Setting Up Cross Domain Tracking
Referral Exclusion List

12. Read More
https://support.google.com/analytics/answer/1034342?hl=en