Difference between registerDefaultNetworkCallback and registerNetworkCallback

I came across registerDefaultNetworkCallback and registerNetworkCallback while updating my Android app for API 28.

Having reviewed the documentation, I cannot find the difference between registering a network callback and registering a default network callback.

When will one use which?

Thanks in advance :)

As far as I understood, the difference between registerDefaultNetworkCallback and registerNetworkCallback it's only based on customisation.
registerDefaultNetworkCallback works (surprisingly) as a default network listener, while registerNetworkCallback it's more configurable. For example:

    val connectivityManager = getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager

    val builder = NetworkRequest.Builder()
    builder.addTransportType(NetworkCapabilities.TRANSPORT_CELLULAR)

    val networkRequest = builder.build()
    connectivityManager.registerNetworkCallback(networkRequest, 
    object : ConnectivityManager.NetworkCallback () {
        override fun onAvailable(network: Network?) {
            super.onAvailable(network)
            Log.i("Test", "Network Available")
        }

        override fun onLost(network: Network?) {
            super.onLost(network)
            Log.i("Test", "Connection lost")
        }
    })

Here onAvailable will be called only if the user connects to a cellular network (by connecting to WiFi it won't log anything). The same does onLost when disconnecting from cellular network.

If we do it like this:

    connectivityManager.registerDefaultNetworkCallback(object  : ConnectivityManager.NetworkCallback() {
        override fun onAvailable(network: Network?) {
            super.onAvailable(network)
            Log.i("Test", "Default -> Network Available")
        }

        override fun onLost(network: Network?) {
            super.onLost(network)
            Log.i("Test", "Default -> Connection lost")
        }
    })

Both functions work as default callbacks when the user is connecting (or disconnecting) to/from a network (it can be either WiFi or cellular).

These are just some very basic examples. Of course NetworkRequest can have a lot of configurations by setting its capability or transportType. You can read more about these in the official documentation of NetworkRequest.