The STACK system is a computer-aided assessment package for mathematics, which provides a question type for the Moodle quiz. In computer-aided assessment (CAA), there are two classes of question types.
Selected response questions In these questions, a student makes a selection from or interacts with, potential answers which the teacher has selected. Examples include multiple-choice, multiple response and so on.
Student-provided answer question In these questions, the student's answer contains the content. It is not a selection. Examples of these are numeric questions.
STACK concentrates on student-provided answers which are mathematical expressions. For example, a student might respond to a question with a polynomial or matrix. Essentially STACK asks for mathematical expressions and evaluates these using computer algebra.