The number of times that a value was set for a variable.
Instances are counted for all hit types, but are not counted when a value is recorded for a variable on a subsequent hit due to persistence.
For example, if a user arrives on your site via example.com, the first image request on your site contains the referrer of example.com. When this value is set, one Instance is attributed to example.com even though this referrer is recorded for all pages viewed during that visit.
Instances for conversion variables in data warehouse were added mid-2011, allowing data warehouse requests to treat a specific conversion variable instance as a metric. These metrics are not available for reporting prior to the time they were introduced.