When a product has more options (size, color, type, material, etc.) does not always exist on stock items for all combinations of options.
Multistep Product Configuration inform the customer that the product does not exist in stock only after it chose all options. It is frustrating for a customer who likes to choose the combination and in the end only to find that the product is not in stock.
Must according to the first option chosen, not to be able to choose to step 2 an option that does not exist.
For example, if the stock no red shirt XL, XL when the box is checked, the next step must at the option not to be red. In this way, the customer can see exactly variants available, without making multiple attempts to find a combination on stock.
This feature is available for the other plugins and useful for products with a lot of options.
and the source