INVOL means that your ticket was reissued involuntarily. Genenerally when a flight is oversold, they ask for volunteers - and rebook them and sometimes give them vouchers. Involuntary means you basically were kicked off the plane b/c whatever reason - oversold - cancelled - etc. It basically just means that if u have to rebook or reschedule - there are no fees or anything - and that the airline screwed up.

Hope that helps