If you recently research Windows 10 IoT with some hardware board such as Raspberry Pi, you should see the .FFU file, and you might realize you can not get enough information from Google search to know the .FFU file information.
Actually, .FFU file is more known by those Windows IoT developers, they download Windows 10 IoT Core package, and they will see .FFU file on the computer folder.
FFU, is Full Flash Update. So now you should acknowledge .FFU file is mostly for a system which targets flash storage such as SD card.
Deploy Windows faster on the factory floor by using the Full Flash Update (FFU) image format. With FFU images, you can apply a Windows image directly to a drive or an SD card, laying down the entire drive at once, including the partition information.
The following is the highlight comparisons among .wim, .vhd and .ffu:
.WIM: For modifying and testing Windows images; File-based imaging style; It includes files only on a partition;
.VHD/VDHX: For deploying Windows to virtual PCs; Sector-based imaging style; It includes full set of drive information and partitions;
.FFU: For deploying Windows on a factory floor; Sector-based imaging style; It includes full set of drive information and partitions;