Testability asks how easily the product can be tested: how much of it can be exercised, how observable it is and how much setup a test needs. A measure relates testable behaviour to the behaviour in scope. Testability has three parts: controllability, observability and the cost of the test. A coverage percentage is a partial measure and can be gamed. A better family of measures includes the proportion of requirements with an automated test, the time to set up a test environment, and the proportion of failures that can be diagnosed from the product's own output. The measure should be tied to the risk, because testing everything to the same depth is not a strategy.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.