This is an archived snapshot of W3C's public bugzilla bug tracker, decommissioned in April 2019. Please see the home page for more details.
The sixth para under 14.7.3 fn:serialize says: "If the second argument is omitted, or is supplied in the form of an output:serialization-parameters element, then the values of any serialization parameters that are not explicitly specified is ·implementation-defined·, and may depend on the context." The last part "may depend on the context" means that either the single-argument version and the two-argument version are context dependent (even if a processor may choose to ignore it and set defaults in another way).