One of the first things you should know about when using and configuring Analytics is how the domain of the web property you are tracking is defined as a “site” for the purposes of viewing your reports. The default setup of the tracking code is designed to make it easy for you to track traffic to a single domain or subdomain (e.g. a single website URL) that does not share user data with other domains or sub-domains.
For example, if your pet store URL is dogs.example.com and you install the tracking code with its web property ID on those pages, tracking is automatically set up to track user data only for this single URL. In a similar way, if you own the primary domain, such as www.example.com, and you install tracking for all pages on this site, user and referral traffic is recorded separately for www.example.com.