The Fixel snippet uses non intrusive tracking mechanism inside the user’s browser. Technically speaking, the user’s data is stored in the Local Storage of the browser. The mechanism of these Web Storage are also documented on the Mozilla Documentation.
Since the Web Storage (Local or Session) is associated with a single domain (root or other), a user switching across several subdomains will have a different ID on each subdomain, and consequently different engagement grades.
For example, if a site consists of www.example.com and shop.example.com, the same user traveling across these subdomains will be identified as two distinct users.
Cross domain support
Similarly, if a site sits on more than one domain, a user switching across several domains will have a different ID on each domain.
For example, if a site consists of mysite.com and anothersite.com, the same user traveling across these domains will be identified as two distinct users.
As rule of thumb, we recommend using a separate Fixel Segment for each domain/subdomain.
If a different domain applies only to a small subset of the site (for example only the payment step during checkout), a single Fixel Segment can be used.
Not sure what’s right for you? Just shoot us an email at firstname.lastname@example.org