"Unsafe" union?

Hi Folks,

The XPath 3.0 specification has this statement:

    The current (second) edition of
    XML Schema 1.0 contains an error
    in respect of the substitutability of
    a union type by one of its members:
    it fails to recognize that this is unsafe
    if the union is derived by restriction
   from another union.

Huh?

What is "unsafe"?

What is that saying please?

The XPath 3.0 specification then says:
    This problem is fixed in XSD 1.1, but
    the effect of the resolution is that an
    atomic value labeled with an atomic type
    cannot be treated as being substitutable
    for a union type without explicit validation.

Again, what is that saying please?

/Roger