Methods, devices and systems for video coding and encoding, which include constraints, restrictions and signaling for subpictures, slices, and tiles, are described. One example method of video processing includes performing a conversion between a video and a bitstream of the video, wherein the bitstream comprises one or more access units according to a format rule, and wherein the format rule specifies an order in which a first message and a second message that apply to an operation point (OP) are present within an access unit (AU) such that the first message precedes the second message in a decoding order.