We went through quite a few stages before so here we go:
- Pre-CCK Era was pretty much like the "Dark Age" when everyone must pay Apple for their MFi Program. We only had just a few portable products (e.g. from Cypher Labs and Fostex etc.) back then, and they're always limited to 16/48.
- Post-CCK Era kicked in when Apple released their Camera Connection Kit. That's really designed for USB Mass Storage in mind for transferring pictures from a digital camera. However, that happened to work for UAC2 (USB Audio Class 2.0) as well and we could connect something like USB DAC or USB microphone to an iPad via CCK. Basically we could go all the way up to 384KHz as well as the multiples of DSD without any issues.
- Post-iOS 7.X Era turned out to be even better since both iPod Touch and iPhone running iOS 7.0 or above could benefit from CCK.
This is an older one for iPhone 4 and iPhone 4S running iOS 7.0 or above
http://store.apple.com/us/product/MC531ZM/A/apple-ipad-camera-connection-kit
This is the latest one for iPod Touch 5G (or later) and iPhone 5 (or later) running iOS 7.0 or above
http://store.apple.com/us/product/MD821ZM/A/lightning-to-usb-camera-adapter
This is an unofficial product from China and it's more convenient as a single-piece solution, of course it might not work anymore once we've updated iOS in the future
http://item.taobao.com/item.htm?id=43233663546