Take a hypothetical example.
A consulting engineer helps a start-up company with the design of a new whiz-bang product. As a start-up, they have little capital to spend for the design up front. The consultant and the company agree to a smaller initial fee, but then a percent royalty will be applied to each unit sold that will go to the consultant over the next X number of years.
How would the consultant be able to verify the number of units sold and the list price of each unit to the amount of royalties collected? Is it 'simple' contract language that forces the company to disclose this information to the consultant? Is it contract language that allows the consultant to audit the company periodically? Does a 3rd party need to get involved?