A method for video processing is described. The method includes performing a conversion between a video comprising one or more pictures comprising one or more subpictures comprising one or more slices and a bitstream representation of the video according to a rule, and wherein the bitstream representation includes a number of coded units, wherein the rule specifies that a decoding order of coded units within a subpicture is in an increasing order of subpicture level slice index values of the coded units.