To recognize users across different devices, what feature must be enabled?

User ID lets you associate a persistent ID for a single user with that user's engagement data from one or more sessions initiated from one or more devices. Analytics interprets each unique user ID as a separate user, which provides a more accurate user count in your reports. To implement User-ID, you must be able to generate your own unique IDs, consistently assign IDs to users, and include these IDs wherever you send data to Analytics. Tip: To set up User-ID, you must first enable the feature in your Analytics account (Edit permission required), and then modify your tracking code.