Coverage closure
Beginner
The long last stretch of testing, when engineers chase down every unticked item on the checklist.
Novice
The late phase of verification in which engineers look at every coverage item no test has reached yet, and either write a test for it, adjust the random rules, or exclude it with a reviewed reason (for example, because it can never happen).
Expert
Iterative: merge coverage across every seed, rank tests by what they add, and sort each hole into missing stimulus, missing checker, unreachable, or unsupported. Every exclusion needs a written reason and a reviewer.
Explained in Verification (Design Flow).
See also: Functional coverage, Code coverage.