The term "trigger" mean when the snowfall have reached to at least the point that you are automatically to plow. For example if you have a 3" trigger, that means if you get 3" or more of snow, you are automatically to plow without customers' request.
In my commercial contracts trigger means only that is when I will start plowing. After that removal is up to me unless specified. This prevents a customer saying that they only want me to plow when they are open and then let 16" pile up overnight. I will wait to plow until 3-4" but then I will continue at this range until snowfall ends.
If making up a contract it is important to cover all your bases. If not a customer will find a hole in your plan.
Since you are the snow removal contractor it is implied that you are the professional in the mix.
A customer that wants to call you when he is ready for you to plow may have no idea of when you should actually start plowing.
The trigger takes them out of that part of the picture.
Time is money and you can't be sitting during a storm because a customer is worried about calling you out too early.
Your job is to assess the storm and be ready to plow with it.
Trigger - Roy Roger's trusted Golden Palomino, which is now stuffed and in the Roy Rogers Museum.
Trigger - activation device on a gun which when pulled causes the hammer to drop, striking the projectile firing pin, which causes a small conatined explosion which in turn expels the projectile out of the gun barrel.
Trigger - Snow removal - snow amount which causes driver to leave confines of warm bed or structure, walk out into severe weather and start truck and keep driving until snow has stopped and pockets are full of Greenbacks.
See three possible definitions.... choose one you like.