Who watches whom, and what each of them has actually proved.
A tick on an arrow is the witness’s own conclusion, fetched from the node that drew it. What makes it worth anything is that you can recompute it without that node — select an edge and check it here, or read every byte on the evidence page.
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Copies of one node, run by one operator, sharing one key and one log. They buy uptime. They prove nothing about each other, which is why they are drawn inside a boundary rather than joined by arrows: reading “three replicas agree” as “three parties verified” is precisely the wrong conclusion.