Select this data source if you've setup a device with xDrip+ as master and decided this other device will be the follower.
A master device is usually the one getting BG from the sensor.
A follower replicates readings from the master device real time (network latency applies).
There is virtually no limit in the amount of xDrip+ follower devices.
See here how to setup xDrip+ Sync follower.