Feedback in a digital learning system has two functions, a pragmatic and an epistemic one. In previous papers, we were concerned with the pragmatic function, showing how digital feedback initiates acting with the system in groups of students. This paper addresses the epistemic function of this feedback by describing how feedback supports understanding algebraic principles. We report on an investigation of learning algebraic rules conducted with a virtual manipulative learning system, distinguishing, identifying and describing different forms of epistemic feedback, that is, how feedback provided by the learning system fosters disclosing and using algebraic principles.