2.1.785 Part 4 Section 188.8.131.52, fieldUsage (PivotCache Field Id)
a. The standard does not restrict the value of the x attribute in any way.
In Office, x is a zero-based index in the parent collection, must be >= -1 and < the total number of cacheField elements. If x points to a cacheField, this cacheField must point back to the same hierarchy the parent fieldsUsage is part of.
b. The standard does not state a dependency between fieldUsage index and cacheField@level.
Office requires that the index of fieldUsage in the fieldsUsage collection determine the mapping between Pivot level and a cache field. The index must match the value of @level in the corresponding cacheField item.