Sub-picture scalable layer SEI message semantics The sub-picture scalable SEI message provides a mechanism for associating a slice group set indicated in a motion- constrained slice group set SEI message with a sub-picture scalable layer. In the following specification of this clause, the terms scalable layer, sub-picture scalable layer, and primary coded VCL NAL unit are used as specified in clause G.13.2.1. A sub-picture scalable layer SEI message shall not be succeeded, in decoding order, by a scalability information SEI message inside the same access unit. When a sub-picture scalable SEI message is present, the following applies: If the sub-picture scalable layer SEI message is included in a scalable nesting SEI message, a motion-constrained slice group set SEI message, which is also referred to as the associated motion-constrained slice group set SEI message, shall be present in the same scalable nesting SEI message and it shall immediately precede the sub-picture scalable layer SEI message in decoding order. The scalable nesting SEI message that contains the sub-picture scalable layer SEI message shall contain num_layer_representations_minus1 equal to 0 and sei_quality_id[0 equal to 0. The variable depId is set equal to the value of sei_dependency_id[ that is present in the scalable nesting SEI message containing the sub-picture scalable layer SEI message. Otherwise (the sub-picture scalable layer SEI message is not included in a scalable nesting SEI message), the sub-picture scalable layer SEI message shall be the first SEI payload in an SEI NAL unit and the NAL unit immediately preceding the SEI NAL unit containing the sub-picture scalable layer SEI message shall be an SEI NAL unit that contains a motion-constrained slice group set SEI message, which is also referred to as associated motion-constrained slice group set SEI message, as last SEI payload. The variable depId is set equal to 0. The slice group set identified by the associated motion-constrained slice group set SEI message is referred to as the 