MNI and SLO

Hello,

I'm reading saml-core-errata-04(sstc-saml-core-errata-2.0-wd-04-diff.pdf),
http://www.oasis-open.org/committees/download.php/22385/sstc-saml-core-errata-2.0-wd-04-diff.pdf

but I'm not sure an intent of "[E55]using the *primary* identifier."
at line 2626.

For example, SPProvidedID is "abc" currently, then SP requests MNI
with "xyz" in <NewID>.
After that  if IdP wants to request SLO, which id should be used in
SLO? "abc" or "xyz"?

Thanks,

--
Hideki