Practice tells you what you can answer today. A retention check tells you what you kept.
What a check is
A check covers one topic and the topics inside it. It asks only questions you have learned, that are not essays, and that you have not answered in the last 7 days. If fewer than 5 such questions exist, it waits a day.
The first check on a topic comes 21 days after your first quiz on it. Change the gap, and how many questions a check asks (10 unless you change it), under Retention checks in Settings. With Scale to a target date, checks fit the days left before your target date.
Taking one
A check opens from 2 in the morning on its day. It shows on Today with Start check. You get feedback on every answer, but a check never moves a question's schedule, and it is scored apart from practice. A finished check cannot be taken again.
The result shows your percent, the check before it, and the questions you missed.
Progress
Progress shows your last checks, retention by topic with the check before, what you have learned, the items most at risk, and the days you studied. A topic whose retention fell is marked.
Retention on Progress comes only from checks. The only place practice accuracy appears is beside retention in the gap between the two, so you can see how much a good practice day overstates what stays.