I believe that the PARALLEL option just modifies the way that the data is inserted to the table. Instead of being inserted above the high water mark in space already allocated to the data segment, a new extent is created. This allows multiple SQL*Loader "parallel" sessions to be run at the same time, each allocating its own extents. So you actually would run multiple SQL*Loader sessions, and the PARALLEL option is just an enabling mechanism for this. If you have a lot of free space above the HWM then this is less space efficient of course.