It marks a detail only the other record has, which the merge is about to add. Every box like that shows ✓ Adding this, ticked to start. Don't want it? Tap it so it says Add this, and that box stays exactly as it was on the card you're keeping. For example, say the file has a birthplace for Grandad George that you know is wrong – tap ✓ Adding this and his card keeps its own.
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