This reference document lists and describes all the dimensions and metrics available through the Real Time Reporting API.
Dimensions
rt:browser
The names of browsers used by users to your property.
rt:browserVersion
The browser versions used by users to your property.
rt:operatingSystem
The operating system used by users to your property.
rt:operatingSystemVersion
The version of the operating system used by users to your property
rt:deviceCategory
The type of device: Desktop, Tablet, or Mobile.
rt:mobileDeviceBranding
Mobile manufacturer or branded name (e.g: Samsung, HTC, Verizon, T-Mobile).
rt:mobileDeviceModel
Mobile device model (e.g.: Nexus S)