The UBIFS image is a file system image.It works on top of a UBI volume.You cannot flash it on the raw flash device */dev/mtd*. You need to create UBI volume first and then update the volume with UBIFS image.
The UBI image is a UBI volume image. It uses the UBIFS image and some other metadata to create a volume on top of a raw flash device */dev/mtd*. In other words you can flash directly a UBI image with standard *nandwrite* command using the '/dev/mtdx' char device.
On CCWi-i.MX51 or CCWi-i.MX53 platforms , u-boot requires UBIFS image, because the internals of u-boot will create the UBI volume and then updates it with UBIFS image.
The UBI image is meant to be used directly by the operating system (Linux/Android), but at the moment Digi use it only on Linux.