What is Proof of delivery?
Short answer
Evidence that an agreed outcome was achieved — a live URL, a timestamped screenshot, a before-and-after of a search page, a visible badge.
Proof is only meaningful when the outcome was defined before the work started. "We did our best" is not a deliverable, and neither is a report describing effort.
At VFCore the proof artifact is also the release trigger: posting it is what settles the payment, and it is what any later dispute is judged against.