Often people describe all hair without a distinctive wave pattern as simply "Type 1". Walker uses the subdivisions to describe how fine to coarse straight hair is. This can be counterintuitive to people, as the rest of the system uses the subdivisions to describe the tightness of wave/curl/coil. I have heard people instead use them to describe if the hair is pin straight or if it tends to flick up at the ends or have a slight bend.
Discussing whether a client's hair is fine or coarse, or if it has a slight bend or flick, is something I do anyway, so I don't find describing straight hair with the Walker system (using either interpretation of the subdivisions) necessary. It is rare for someone with straight hair to be aware of the system. And if they are, they could be using Walker's original definitions, or the reinterpreted definitions. This adds confusion, rather than bringing clarity and mutual understanding, which is the goal of using a system such as this.