You can check the VPU Application programming reference for usage and programming the VPU, and
this is a data structure for configuring MJPEG-specific parameters in encoder applications.
mjpg_sourceFormat is the chroma format. The format means chrominance size of source image and can be a value
between 0 and 4: 0 = 4:2:0, 1 = 4:2:2 horizontal, 2 = 4:2:2 vertical, 3 = 4:4:4, 4 = 4:0:0.
But, it never said about the h264 I guess it don't but you can check the manual.