This table is an indirect extension to the quiz_questions table, because the quiz_question_datasets table can link a question to one or more datasets. Each dataset represents a variable, that is used either in the questiontext or in the answer to a dataset dependent question.
Dataset items can be created for each dataset. The quiz_dataset_items table stores these possible values for the variables defined in the quiz_dataset_definitions table.
The quiz_question_datasets table is used by dataset dependent questionypes (i.e. calculated) to link datasets to questions.