Sentinel is a PC application that you program the scanner with and update its database and firmware (operating system).
Siren is a mobile application (currently only for iOS, but soon to be for Android as well) which displays the scanner's screen on your mobile device (and optionally sends audio from the scanner to your device).
Siren is only for the BCD536HP. It has the wi-fi device needed for the Siren app. The BCD436HP has no wi-fi.