2.1.1509 Part 1 Section 21.2.2.128, order (Order)
a. The standard states that the values of the val attribute are defined by the XML Schema unsignedInt datatype.
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Office restricts the value of this attribute to be at most 2147483647.
b. The standard does not have any uniqueness restrictions on the order element.
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Office requires the value of this element to be unique amongst any other order elements of the same collection containing the parent series element.