An example of a "mileage run":
SYD-ADL in J on a direct flight costs me $A642 and gets me 1250 QF FF points and 30 status credits (the minimum applied per sector).
SYD-CBR-MEL-ADL in J costs me $A649, gets me 3750 QF FF points (minimum 1250 per sector) and 90 status credits (30 per sector).
So doing SYD-CBR-MEL-ADL is considered a "mileage run".
Dave