The thunderbolt 3 (Usb-c) to thunderbolt 2 adapter lets you connect thunderbolt and thunderbolt 2 devices — external hard drives and thunderbolt displays.
The Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter lets you connect Thunderbolt and Thunderbolt 2 devices such as external hard drives and Thunderbolt docks to any of the Thunderbolt 3 (USB-C) ports on your MacBook Pro.
As a bidirectional adapter, it can also connect new Thunderbolt 3 devices to a Mac with a Thunderbolt or Thunderbolt 2 port and macOS Sierra
The Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter is for 2016 MacBook Pro models, and isn’t compatible with MacBook (Retina, 12-inch, Early 2015) and later
Note: This adapter supports Thunderbolt 2 devices and Thunderbolt displays. It does NOT support DisplayPort displays like the Apple LED Cinema Display or third-party DisplayPort and Mini DisplayPort displays.