Who tests what.
Eight handoffs, each with its own tests. Pick a handoff to see the test, the lab, the method, who pays, and who settles a disagreement, or switch to the grid to see the whole chain at once.
The specification is the referee. A malt lot that meets the written specification is bought, so the buyer writes the specification out line by line before the run. Testing is paid for by the party that orders the grain, built into the price. Grain-side disputes have an answer: an official sample split four ways. Everywhere else, the rule is to put it in writing before the lot moves.
Reading the matrix
Each handoff is a place the gluten-free claim and the lot's quality can break. The test at that point is only worth something if someone with no stake in the answer runs it, on a properly drawn sample, and a retained sample exists to settle any argument later.
The gluten-free claim itself rests on the evidence gathered before fermentation. Fermentation breaks gluten into fragments that common tests can't reliably read, so the finished-beer test confirms the lot is sound and doesn't prove the claim on its own.