I've only seen UA issue a ticket with INVOL if they rebook you on another carrier and it is in a different class of service than originally ticketed.

I've had it happen three times and in all instances I was travelling on a coach fare with upgrades confirmed check-in. The flights were subsequently cancelled and they rebooked me on another carrier not in coach but in F.

I believe they only do this for their top level elites and not every agent will do it. Most agents will simply return your upgrade certificates and endorse the original coach tickets over to the accepting carrier.

But when the agent do decide to let you keep your upgrade, new tickets must be issued since you can't endorse the original tickets up to a higher class of service. These new tickets will bear INVOL for the fare code.

I guess they do it this way to prevent you from trying to get a refund for F from the accepting carrier.