To explain that the SCN HEADROON, we must first explain the SCN. The SCN is a sequence of life of database or as many authors says a Oracle’s clock, that increases every second or every time a commit occurs. This sequence is continuously increasing, however, how far can it increase?, this value is usually determined by power form 2exp42. This is a limit which has the database and would be of the order of trillions. The SCN HEADROON is defined as the space between the current SCN and this limit.
HTH – Antonio NAVARRO