As best as I can tell, the defining aspect of a system is that the knowledge of one card will allow you to determine the value of the next card in the stack based on a rule. The Si Stebbins deck is a simple system, and the Osterlind Breakthrough Card System is a much more ingenious system. In both cases you use a rule to determine the value of the next card in the deck. The rule can be mathematical or it can even be rhythmic such as the Eight-Kings verse.
Granted, you can memorized any of the systems and have it be a true memorized deck. After that, the rules are irrelevant, other than a backup method for the "next card" analysis. However, a system's original design is intended to be a determination of the next card in the deck.
A memorized deck (such as Joyal, Aronson, Tamariz, etc.) have no rules other than those used to memorize the deck. Aronson and Tamariz use general mnemonic techniques to get to your memorized knowledge. Joyal designed his memorized deck in such a way so that some simple rules can be used only during the memorization process. None of these rules will help you determine the next card in the stack in the same way that a card system can. you just have to know your memorized stack.
Again, the key distinction for me is whether the stack was designed so that you can determine the next card in the stack, or whether you just have to know every card's stack value.
Nick Pudar StackView Version 5