The Nikon unified flash control allows the camera and flash unit to share settings. If a flash unit (or in this case the i-TURTLE TTL converter) that supports unified flash control is mounted on the camera, changes to flash settings made with the compatible camera (e.g. Nikon D850) are reflected on the flash.
If you choose the new i-TURTLE unifie TTL converter for your Nikon DSLR camera which is compatible with unified flash control, the flash control mode, flash level, and other flash settings can be adjusted using the Optional flash item in the Custom Settings > Bracketing/flash menu of your Nikon D7200 (latest firmware!) camera, or using the Flash Control Mode item in the Photo Shooting > Flash control menu of the Nikon D850 camera. It means you can easily change between manual or TTL modes under water while you don’t have to change the settings on your underwater strobe.