Source：http://support.apple.com/kb/HT6331
Each of these diagnostic capabilities requires the user to have unlocked their device and agreed to trust another computer. Any data transmitted between the iOS device and trusted computer is encrypted with keys not shared with Apple. For users who have enabled iTunes Wi-Fi Sync on a trusted computer, these services may also be accessed wirelessly by that computer.
1. com.apple.mobile.pcapd
pcapd supports diagnostic packet capture from an iOS device to a trusted computer. This is useful for troubleshooting and diagnosing issues with apps on the device as well as enterprise VPN connections. In iOS 8 and later, this capability is restricted to the USB interface and can no longer be accessed over Wi-Fi. You can find more information at developer.apple.com/library/ios/qa/qa1176.
2. com.apple.mobile.file_relay
file_relay supports limited copying of diagnostic data from a device. This service is separate from user-generated backups, does not have access to all data on the device, and respects iOS Data Protection. Apple engineering uses file_relay on internal devices to qualify customer configurations. AppleCare, with user consent, can also use this tool to gather relevant diagnostic data from users' devices. In iOS 8 and later, this capability requires additional configuration before use.
3. com.apple.mobile.house_arrest
house_arrest is used by iTunes to transfer documents to and from an iOS device for apps that support this functionality. This is also used by Xcode to assist in the transfer of test data to a device while an app is in development.