User-ID and Cross Device
User-ID is a Universal Analytics feature that you can use to associate multiple sessions (and any activity within those sessions) with a unique ID. With it, you can get a more accurate user count, analyze the signed-in user experience, and get access to the Cross Device reports. Learn more about Universal Analytics
The Cross Device reports give you the tools you need to organize data across multiple devices into a cohesive analysis, so you get a better idea of how different devices, sessions, and interactions are connected to each other.