Following options are available to define which packaging level can be
sold and which product can only be sold by packaging.

- Sell only by packaging: On product template model, this checkbox
  restricts sales of these products if no packaging is selected on the
  sale order line. If no packaging is selected, it will either be
  auto-assigned if the quantity on the sale order line matches a
  packaging quantity or an error will be raised.
- Force sale quantity (on the packaging): force rounds up the quantity
  during creation/modification of the sale order line with the factor
  set on the packaging.
