Make the most of the opportunity
Whether you were called as a 'source' or a 'prospect', you have a unique opportunity to build a relationship with the retained executive search firm, so make the most of the opportunity by being as helpful to the recruiter as you can.
A source is someone who can be of value to the search firm in terms of directing them to suitable candidates, getting a good lead, or offering market, industry or company insight. A prospect is someone who the retained search firm has identified as a potential fit for the position. The search firm contacts them to find out more about their qualifications and experience, and to gauge interest in a potential opportunity. Even if the call amounts to nothing, the information and help you have given will keep you in good standing for future calls.