When you click Alarm Cancel, sounders and sirens signaling the alarm are silenced and the status of the sensor(s) that caused the alarm is reset (cleared).  

On a system control panel, an authorized user must enter their user code once to silence
the sounder in the panel and any external sirens.

Then, to completely clear the alarm, the user must enter their user code a second time. This clears the data sent by the sensors about the condition that caused the alarm in the first place.
When Alarm Cancel is clicked/selected on the Remote Service website or the app, the logged-in
person’s User Code is sent to the security panel twice

Sending the user code twice duplicates the actions required to completely cancel an alarm from the system control panel itself.