another classic question which the answer exist if u used the search function
valet is to enable the alarm to trigger when the boot or bonnet is opened... means when u valet park, u don't want the person parking your car to open ur boot or bonnet right? so u enable it valet mode.
even when the alarm is disarmed, only car doors can be opened, if bonnet or boot is opened, the alarm will sound...
it left your car's engine runing while you armed your ... i think :hmmmm:
my car has one too... useful if i wanna dash to buy something but don't want to turn off the engine convenient for turning the aircon on during hot days
... or for quick getaway after robbing a bank