2.1.305 Part 4 Section 2.11.3, endnote (Special Endnote List)

a.   The standard allows any integer for footnote or endnote ID.

Word only allows integers between -2147483648 and 32767.

b.   The standard says the footnote or endnote ID is used to match the ID in the footnoteRef or endnoteRef, although footnoteRef and endnoteRef do not contain an ID.

In Word, the footnote or endnote ID is used to match the ID in the footnoteReference or endnoteReference.