Every phone has only 1 bluetooth device, but there might be several applications that want to transmit ibeacon signals. Which signal will be transmitting? The application that was launched later?
The application on the foreground.
From Apple docs:
Any iOS device that supports sharing data using Bluetooth low energy can be used as an iBeacon. Because the app you write must run in the foreground, iBeacon support on iOS devices is intended for testing purposes and for apps that always run in the foreground anyway, such as point-of sale apps. For other types of iBeacon implementations, you need to acquire dedicated beacon hardware from third-party manufacturers.