decimal vs double value spaces

Would someone kindly enlighten me as to the difference in
the value spaces of xs:decimal's and xs:double's.

I understand that the former doesn't lexically accept exponent
notation (E+02 e.g.) but note that some (possibly unfinished/
possibly naive) Java XPath engines internally represent xs:decimal and
xs:double as Java doubles thereby reducing the (internal) distinction
to nought.