Member systems are logical collections of member parts that maintain the design basis and physical alignment of the member parts for analysis, design, and manufacturing. For example, in vertical cross-bracing, typically one of the vertical braces is split into two parts so that it does not interfere with the other vertical brace in the cross. The member system for that split vertical brace maintains the co-linear alignment of the two parts when you move either outside corner of the vertical brace. Another example of a member system would be a jacket leg. The leg is comprised of different parts, including cans that have different cross-section sizes, but you want the entire leg to move as a single member.