Crossing Over refers to:
changing realities
changing our physical positions in this reality
shifting consciousness (meditations, dreams, etc.)
working with chromosomes
changing something major in our lives
crossing the line
changing thought patterns
moving from one website to another, etc.
Crossing over often is associated with bridges. Bridges connect grid realities through time and space and something else science is about to discover. They unite things that cross back and forth. Your soul crosses back and forth all the time. It is doing it right now as you sit there reading this helping you understand that the soul has multidimensional experiences. Enjoy the adventure.