2.1.753 Part 1 Section 220.127.116.11, field (Field)
a. In the standard for the element description it says there will be as many <x> elements as there are item values in any particular column or row.
Office says that there will be as many <field> elements as there are fields in the column or row axis.
b. The standard notes that elements may not be explicitly written and i elements, @r and @v used, which do not occur on this element or its parents.
Office assumes that field elements are always explicit and the x attribute is required.
c. The standard specifies that the x attribute is the index to a pivotField item value and there will be as many x elements as there are item values in any particular column.
Office specifies that the x attribute is a valid zero-based index to a pivotField element in the pivotFields collection with value equal to -2 or greater than or equal to zero and less than the element count in the pivotFields collection.
d. The standard does not restrict the document from having duplicate field indexes (@x).
Office requires that the value of @x cannot be duplicated within the parent element.