… Multiple simultaneous referents:
link

ref points to a link that links the persons of interest

presidents Adams were born in Massachusetts
...
Adams, John Adams, John Quincy
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If you have two or more people who are frequently grouped together, you may also use the link element to group them. If you define the group as one link within a linkGrp, you can then use ref to point to the xml:id of the link, rather than to the two separate entries in the personography.

Contextual Nuances, slide 8 of 11