On the one hand, the context is different. An affordance, as you point out, signals how we should use an item. In a way, it suggests a meaning. A kanban's meaning is typically defined as the process (largely) is known.
In the story about Milk Kanban, the index card is self-explanatory, but that's just an extra bit, to make the process more bullet-proof.
On the other hand, if we generalize both examples, we land in a place where the design (of things, of processes) should communicate how to act. I either interact with an object or I do my action within a process.
BTW: The Design of Everyday Things is another great reading recommendation in this thread.
On the one hand, the context is different. An affordance, as you point out, signals how we should use an item. In a way, it suggests a meaning. A kanban's meaning is typically defined as the process (largely) is known.
In the story about Milk Kanban, the index card is self-explanatory, but that's just an extra bit, to make the process more bullet-proof.
On the other hand, if we generalize both examples, we land in a place where the design (of things, of processes) should communicate how to act. I either interact with an object or I do my action within a process.
BTW: The Design of Everyday Things is another great reading recommendation in this thread.