Actually it seems to me that RW can be expanded upon a bit. It is not just belly flyers. It is anytime 2 or more skydivers or canopy pilots build formations or work in close proximity. If it is canopies then it is Canopy Relative Work (CReW), freeflyers often to vertical Relative Work (vRW). Jessica also mentioned wing loading ratios. That is the most common ratio talked about with canopies but aspect ratio is also worth understanding. It is the relationship of the cord to the span of a canopy (length to width). Brian