Specific limits or warning signs you set for your marketing tests. If these limits are crossed, it tells you there's a problem, and you should probably review or stop the test.
Testing is great, but it can turn into a money pit if you don't have guardrails. By setting a "stop loss" (like a max CPA), you allow yourself to experiment without risking the company's future. It turns marketing from a gamble into a calculated, safe process.