2.1.516 Part 1 Section 17.16.17, ffData (Form Field Properties)

a.   The standard says that if ffData is omitted, the properties associated with the form field shall be based on default values for the form field.

In Word, if ffData is omitted, the form field is considered invalid and cannot be used or edited.

b.   The standard states that label and tabIndex are child elements of this element.

Word ignores these elements and does not round trip them.