2.1.1905 Part 4 Section 188.8.131.52, Guid (GUID)
a. The standard states that this element is a unique identifier for the source.
When copying a source entry from a source document to a destination document, Word checks to see if the destination document has an entry with the same tag (“[ECMA-376] Part 4 §184.108.40.206; Tag (Tag)”) as the entry being copied. If a match is found, Word then compares the GUID of the matching entry in the destination document with that of the entry being copied. If the GUIDs are identical, or if the destination document has an entry with the same GUID as the entry being copied, Word assumes the source entry to be already present in the destination document and no new entry is created.