Firmware

iTunes contacts ax.phobos.apple.com to check for updates.

Apple TV
Apple TV firmwares have two different version and build numbers, which can lead to confusion when referencing them. The internal build number is the best way to refer to a specific firmware, since it can't get confused for another number.
 * The "Marketing"/"Displayed" information is what is reported on the Apple TV's About screen.
 * The "Internal" information refers to the "ProductVersion" and "ProductBuildVersion" keys in the IPSW's BuildManifest.plist.