You can compare it to writing software. When written on assignment they receive all the right and you the fee for producing it, when its a private development, they can BUY the rights to both the final product and the source-code. In both cases you ALWAYS retain the artistic rights as it has sprung from your mind. I recall a project where the client got a handwritten product at an hourly fee (costing them a few 1000 bucks) and wanted the source. They could obtain the source-code at a mere $64,000. They gracefully declined. Hope this helps. Maxx