The customer must have appreciated the collaboration with you. Only then will the customer appreciate the value that your continued influence will have on their business, either through the work itself or through access to your know-how. Although they are not technically retained, they are often related to continuous work and refer to the period during which you are (or not) paid for this work. Let me try a stupid question. Honestly, I studied philosophy and became a professor of philosophy. Only two years ago, I was interested in economics, I did an online course at Harvard Business School (Leading with Finance), and now I am doing an online MBA from a high-level American university. I am inclined to analyze financial analysis and I like debates about change management. I want to become a consultant and I think clients are looking for my expertise in diagnosing their financial health business and/or processing organizational change.