Simply put the new smart app will allow you to use non ADT sensors with the ADT alarm system.
The new app allows you to setup a selection of sensors to monitor and send out self monitored alerts of activity if something is triggered while the ADT system is alarmed and by which armed state it is in.
It doesn’t really help if you want everything to be monitored by ADT, but if you have a bunch of sensors before getting the ADT panel this will allow you to use them the same way you could before the ADT panel in a self monitored way, but base the trigger of an alert on weather the real alarm is armed.
Think of it like this. Joe has been using smartthings with SHM for a year and has everything monitored with standard z-wave sensors. He is tired off fighting with some of the quirks of SHM and its partial attempt at functionality. So he goes out and gets the ADT Smartthings panel from Best Buy for a full blown alarm. When he gets home he realizes that it works fine with his 2 contact sensors and 1 motion sensor in the package, but his z-wave sensors aren’t talking to the alarm side. Now with this new smart app it will enable him to use everything he had deployed with SHM and allow them to trigger unmonitored alarms based on the ADT alarm status. It will send out notification, turn on and off the siren/strobe if you have one, flash lights. You can even create zones by loading the app multiple times with different sets of sensors.