This field will only be visible from within the cart page. If your customer has selected a one-time purchase product, that can also be purchased as a subscription, they will see the following clickable text (highlighted below).

Once your customer clicks on the text (highlighted above) they will be presented with the delivery options for that product. The last option available is what this field corresponds to. When your customer clicks Unsubscribe the delivery options will close, and the product will remain as a one-time purchase.