What’s different about mastering for digital?
Digital formats like mp3, AAC (iTunes) etc employ “lossy” compression, meaning they achieve smaller file sizes by removing and approximating some audio data. While modern encoders are generally pretty good, there are still some issues with frequency response, headroom, and intersample clipping that need to be addressed. While you can just rip directly off a CD to mp3, you’ll get a better quality final product if your material is mastered specifically for digital. This includes testing against encoder types (how does it sound as an AAC? As an mp3? At various bit rates?), verifying headroom, checking for intersample clipping, etc.