processing instruction with "xml" target

[Ari Krupnik <ari@cogsci.ed.ac.uk>]

"The target names "XML", "xml", and so on are reserved for
standardization in this or future versions of this specification."
(http://www.w3.org/TR/REC-xml/#sec-pi)

Tim Bray annotated this with "names beginning with the letters x, m,
l, are "reserved", whatever that means."

Is there an accepted understanding of what "reserved" means in this
context? Is it reasonable for a processor to treat PIs with "xml"
targets as a well-formedness errors?

Ari.

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