2.1.757 Part 1 Section, 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.