scatter/gather list support

[Kylie Liang]

On http://www.freebsd.org/doc/en/books/arch-handbook/scsi-general.html,  it mentions " Actually, it	  seems like the scatter-gather ability is not used anywhere	  in the CAM code now.  But at least the case for a single	  non-scattered virtual buffer must be implemented, it is	  actively used by CAM.". 
 
What does it mean? How does CAM level handle scatter-gather I/O request? Thank you.