Viability relates to the underlying business models: whether or not you have a commercially sound proposal. For example, before launching the product you want to know how large your potential market is and what people are willing to spend. Although asking people through surveys if they ‘like it’ or  ‘are interested to buy it’ offers some insight, it is very unreliable. What people say and what they do can differ greatly, especially when it concerns spending money. Therefore, genuine viability testing allows people to ‘vote with their wallet’. That is: whether they are spending money.

You need specific prototypes in order to do such tests. Examples of testing retail formulas are pop-up restaurants and stores. Or selling small series of a new product in 'test markets', rather than all over the country. Another example is inviting a limited number of visitors to an event to test whether people like it before ramping it up to the intended scale.

As viability relates to the business model you may want or need to test cost-related matters. For example, if you plan to sell your products online, you need to know what packaging is needed to get it delivered in perfect condition and what the cost is. This also requires you to create prototypes.