Google Analytics Custom Variables Usage Statistics

Custom variables are name-value pair tags that you can insert in your tracking code in order to refine Google Analytics tracking.

https://developers.google.com/analytics/devguides/collection/gajs/gaTrackingCustomVariables?hl=en

Analytics and Tracking

Google Analytics Custom Variables Customers

Get access to data on 31,211 websites that are Google Analytics Custom Variables Customers. We know of 11,486 live websites using Google Analytics Custom Variables and an additional 19,725 sites that used Google Analytics Custom Variables historically and 3,188 websites in the United States.