Possible states of student submissions.

SUBMISSION_STATE_UNSPECIFIED No state specified. This should never be returned.
NEW The student has never accessed this submission. Attachments are not returned and timestamps is not set.
CREATED Has been created.
TURNED_IN Has been turned in to the teacher.
RETURNED Has been returned to the student.
RECLAIMED_BY_STUDENT Student chose to "unsubmit" the assignment.