Glossary
Every term here carries one liftable sentence, the mistake people make with it, and how we report it. Written so a definition still makes sense pulled out on its own.
The vocabulary in this field is unusually slippery. A soft bounce is not a small hard bounce, catch-all does not mean valid, and unknown is a result rather than a failure to produce one. Most arguments about verification turn out to be arguments about which of these words two people meant.
Run a sample through it
100 free checks a month, no card, and the unknowns come back labeled.