Output is the submission body of an OUTPUT problem: a file uploaded for each of the problem's inputs (see TestingService.ListInputs).

A test is named by id and not by index, which is unique only within a testset. The index is frozen when the submission is created, so it still renders after the tests have been renumbered.

Quiz is the submission body of a QUIZ problem: the answers given to the questions of the quiz.

A question and the options picked in it are named by id, which is what grading resolves them through. Answering only needs those ids: the positions next to them are filled in by the server and frozen when the submission is created, so the submission renders on its own — "Question 3, answer B" — after the questions have been edited or deleted.

Widget is the submission body of a WIDGET problem: the answer composed inside the problem's widget (see WidgetService), as a JSON document of the widget's own design.

The platform does not read inside the answer. It is stored as sent, so the widget can show it again, and graded by the problem's checker as the output file of every test, the way an output-only submission is.