First, thanks for checking out the iOS cast sample and that is a very good question. The part FFE417E5 in the key is default Bitmovin CAF(cast) receiver application ID. This web receiver application is built and deployed by Bitmovin to allow customers to quickly get up and running and start testing cast functionality. This receiver is based on Bitmovin CAF receiver reference code.
For production purposes, customers are in general advised to build their custom CAF receiver applications based on Bitmovin reference and Google documentation and register the same at Sign in - Google Accounts
Bitmovin CAF tutorial provides information on how to use the custom CAF receiver app Id from different Bitmovin sender SDKs(Web, Android and iOS).