"INVOL REROUTE" is just the terminology used in the reservations system to indicate that your PNR/itinerary has been modified by airline staff for whatever reason - so if another agent pulls up your SFO-PHL-MCO ticket and finds that you've been INVOL REROUTEd on a direct United flight SFO-MCO, they will know that it's because "US186 CANX." It's not a contract of carriage term.