Where do union and sovereignty originate from?
Union is an internal experience, not an external one.
We can, then, interpret external experiences of union (for example, in relationship) as evidence of our own capacity for creating the same experience inside ourselves. We literally wouldn't be able to have the external experience if it wasn't generated from within. Other people cannot "do" union to us.
Sovereignty's role here has to do with attachment. This means holding the wisdom that externalized experiences we might otherwise be inclined to accredit to a specific person or environment necessarily live inside of us, and letting go of the grip of control, the label, the needing to know or to understand.
Being sovereign in this case looks like non-attachment.
But to be clear, non-attachment is not the same thing as detached, avoidant, or aloof. Non-attachment truly is just presence (with what is, not what is not).
And here's where it gets fun: presence is how you access union (within yourself, and with others).
(all things in the universe are circular, not linear...)