In article , Mike
Barnes writes

I see, thanks. I'm used to even more basic configurations where opening
the door merely starts a timer and further action is required to disable
the alarm.

The idea is to get it sounding early to discourage entry. There'll be a shock
sensor on the door frame too for an early trip and a v loud sounder right by
the door, again to discourage entry. Arming off the lock means there's no
excuse for (the tenants) not setting the alarm. It's a 2nd storey flat so the
door is the most likely entry point.
--
fred