Every piece shows the sources it stands on and an Ed25519 signature. You can re-check that signature in your own browser. Nothing is sent to a server to do it.
A signature proves two things: who filed a piece, and that it has not changed since. It does not, and cannot, prove that a statement inside is correct. You can sign a mistake. The receipt says who is accountable for a piece and that it has not been altered.
The receipt is accountability, permanence, and a name on every key. It is added trust, never a replacement for judgment, and never a claim that a fact is true.
On any story, a reader can open the provenance panel and re-run the hash in their own browser against the served bytes. If a piece had been altered, the check would fail. See it in action on any published paper.