Hello,
The "length of collector assembly" is simply the length of the SCA. The description in Help is confusing, and I will revise it for the next version of SAM.
The "piping distance between assemblies" is the length of pipe that connects two adjacent SCAs in a loop.
In the loop configuration diagram on the Solar Field page, the length of collector assembly is represented by the length of one of the rectangles representing a collector, and the piping distance between assemblies is represented by the length of the red or blue line that connects two of the rectangles.
Best regards,
Paul.