App+Web properties provide robust way of reporting on cross-platform analytics.The new property provides two ways by which we can identify users in the standard reports: by custom User-ID or device.
By device: Upon choosing this option, the web stream data would capture analytics cookie ID and app stream data would capture App-Instance ID, to identify users.
By User-ID and device: This is a default option available when we set up the property. This option identifies users based on the custom identifier (like email hashed or unique id) which we have to set-up. If no User-ID is collected then users are identified by device method explained above.