The only time "Top 8" becomes relevant is for seeding the NCAA postseason tournament. The Top 8 are national seeds and would host the first two rounds at their home stadiums. The body which determines that is the NCAA Softball Selection Committee. The primary tool used is the RPI, and as a general rule the committee uses the RPI as a guide to select the seeds, though they don't always follow it to a tee.
NCAA Softball RPI
NCAA Softball RPI