OptionalbyThe items count by position, not by index.
The field, by index; -2 the values.
OptionalitemsBy index into the field's items, or into the data fields for -2; absent, all of them.
OptionalrelativeThe items are relative to the cell.
Optionalselectedfalse: the items are not selected; absent, they are.
OptionalsubtotalsWhich of the field's subtotals, as OOXML spells them (defaultSubtotal, sumSubtotal, ...).
The items of one field a PivotAreaJSON picks.