I went to Google.
I searched on RoHS and on RoHS:YL
I found:
http://www.rohs.gov.uk/
http://en.wikipedia.org/wiki/RoHS
http://europa.eu.int/eur-lex/pri/en/oj/dat/2003/l_037/l_03720030213en00190023.p df
(the original ROHS treaty in PDF format)
and as for the Y and YL additional notations, there was...
http://sunsolve7.sun.com/handbook_pub/Systems/common-docs/RoHS_Communication.ht ml#meaning
which is linked directly from the V490(RoHS) page in the Sun System Handbook.
http://sunsolve.sun.com/handbook_pub/Systems/SunFireV490_R/SunFireV490_R.html
It all means that the systems are now being manufactured with materials
that are less detrimental to the environment if you were to throw them into a landfill.