If you've implemented the location callbacks for a specific region (defined by a UUID), then you'll receive messages with that detail how far you are from the device (within about 70m, depending on your device). Depending on the device, the power settings, etc, this can be very accurate.. to < 1m. The IOS api's also support background processing so the app doesn't necessarily need to be in the foreground to process. You will get called back when you enter and exit regions.