Methods, apparatus, systems for video processing, including video encoding or video decoding are described. One example method includes determining an output sub-bitstream by extracting a sub-bitstream of one or more target subpictures from a bitstream of a video including video pictures. At least one of the video pictures includes one or more subpictures. The output sub-bitstream conforms to a format rule specifying that the one or more target subpictures is represented as a single subpicture in the output sub-bitstream.