A CDN Selection developer designs and implements algorithms to choose the best Content Delivery Network (CDN) based on factors like network conditions, server load, and geographical location to ensure optimal content delivery speed. They also develop systems to automate this selection process. A CDN Federation developer, on the other hand, works on the collaboration of various CDN providers to extend their reach and improve content delivery. They develop and manage systems that enable multiple CDNs to function as a single, unified global network, ensuring seamless content delivery regardless of the user's location.