Motivations for restricting the "all" group

I have been trying to do some research into regular
expressions and the "all" group used in XML Schema, as
mentioned in

http://www.w3.org/TR/2001/REC-xmlschema-0-20010502/

It is clear that "all" is in some sense restricted, since it can only
combine elements, and must appear at top level. My question is, what was
the original inspiration for "all", what is the "unrestricted" form of
"all", and why was it originally restricted, in the design of XML Schema?

Some academic authors claim that "all" is a restricted form of
"interleaving", a known operator in regular language theory, for which
the membership problem is NP-complete. But this is not clear to me, as
interleaving means that the words are shuffled in a way that does not
seem to make sense for natural languages. I believe that the "&" from
SGML is a more natural extensions, but I cannot find any reference to
the original motivation, or to the reasons for limiting "all".

Thank you for any help,

Dag Hovland