The "Definition of the S-Coordinate for graphical plots along the LCLS beamline", is at http://www-ssrl.slac.stanford.edu/lcls/prd/1.1-103-r0.pdf
The SUML is the beam path length (as the "crow" flies), including all
curves and bumps along the way.
...
These coordinates are described in an LCLS tech note at:
http://www-ssrl.slac.stanford.edu/lcls/technotes/lcls-tn-03-8.pdf
The length of a device is included in the EXCEL MAD-output file...
http://www-ssrl.slac.stanford.edu/lcls/linac/optics/lcls.xls
...under column "F", called "L_EFF". For magnets, this is the magnetic
length, not the physical length.
...
The "LCLS" Z-coordinate is used in the "LTU" EXCEL file at:
http://www-ssrl.slac.stanford.edu/lcls/linac/optics/ltu.xls
The index page with links to these EXCEL files notes which coordinates
are used ("linac" or "LCLS"):
See http://www-ssrl.slac.stanford.edu/lcls/linac/optics/.
I know it's a bit much, but that's the story.