2.1.1501 Part 4 Section, idx (Index)

a.   The standard places no restrictions on the val attribute beyond type information.

Office requires that the value of the val attribute be unique amongst idx elements within the same collection.

b.   The standard states that the values of the val attribute are defined by the XML Schema unsignedInt datatype.

Office restricts the value of this attribute to be at most 2147483647.