The orthodox description of a kanban is a rectangular card in a plastic sleeve used to reorder materials from a supplier or an upstream process, or a triangular metal plate used to signal production for a process that requires changeovers for lot production.
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The tile of section 3 of chapter 4 from the first textbook on the Toyota Production System is titled “This Too (is a kanban)”. The author (possibly dictated by Ohno, possibly written by Fujio Cho, it’s not clear) states that as long as the item acts as a visual display and performs certain functions, it can be a kanban. Three examples are given, as summarized below:

Carts can be kanban. When a set number of carts containing a set quantity of parts of a set item is moved between set locations based on set rules, this can be kanban. Simply, when there are a certain number of empty kanban carts, the upstream process takes these and brings them back full…