2.1.840 Part 1 Section 18.14.8, externalLink (External Reference)

a.   The standard states that the extLst child element is mutually exclusive with the externalBook, ddeLink, and oleLink child elements.

Excel allows an extLst child element to follow an externalBook, ddeLink, or oleLink child element.

b.   The standard states that externalList element can be empty.

Excel requires that an externalList element must contain one of externalBook, ddeLink, or oleLink.