This is a folder used on Apple Internal firmwares only. It contains bundles and applications used for testing, like SkankPhone or SwitchBoard.app. Its content is automatically "unified" with the respective filesystem folders. For example, if there is a folder
/AppleInternal/System/Library/CoreServices its content will be unified with
If the file
/AppleInternal/System/Library/CoreServices/SpringBoard.app/SpringBoard exists, when the system attempts to load
/System/Library/CoreServices/SpringBoard.app/SpringBoard, it will really load the file out of the /AppleInternal folder instead. All of this is transparent to the other applications as it is all handled in the kernel.
Varies, but can include: