You’re flying to a destination for a purpose such as family gathering or a business meeting, and even though you made to the airport on time, the flight was delayed because of reasons in-hand of the airline and you will not be able to attend that event. This is when you must call out “Trip in vain”.
There is no official policy outlined by any authority, but most airlines will honor this if you ask. Airlines have their own policies to compensate you, but seeing from examples like this, this and this, you should be able to get something out of the airlines.
Basically the rule says that: If, for some reason, something happens to the flight that’s within the airline’s control and the passenger misses the entire purpose of the trip, the airline is entitled to compensate the passenger or provide an alternate solution.
Although this can be applied to business and personal trips, it’s almost at the discretion of the customer relations agent on the phone or the desk agent at the gate.