ity layers. The priority layer information SEI message shall not be included in a scalable nesting SEI message. Rec. ITU-T H.264 (01/2012) pr_dependency_id specifies the value of dependency_id for the VCL NAL units for which alternative values for priority_id are indicated. num priority specifies the number of layer representations with dependency_id equal to pr_dependency_id for which alternative values of priority_id are indicated. alt_priority_id| i ] specifies the alternative value for priority_id for the VCL NAL units of the primary coded picture that have dependency_id equal to pr_dependency_id and quality_i equal to i. The layer representation of the primary coded picture with dependency_ic equal to pr_dependency_id and quality_id equal to i may or may not be present in the access unit. G.13.2.5 Layers not present SEI message semantics The layers not present SEI message provides a mechanism for signalling that NAL units of particular scalable layers indicated by the preceding scalability information SEI message are not present in a particular set of access units. In the following specification of this clause, the terms scalable layer and primary coded VCL NAL unit are used as specified in clause G.13.2.1. A layers not present SEI message shall not be included in a scalable nesting SEI message. A layers not present SEI message shall not be present in an access unit that does not belong to the target access unit set of any scalability information SEI message. A layers not present SEI message shall not be succeeded, in decoding order, by a scalability information SEI message inside the same access unit. The set of access units consisting of the access unit associated with the layers not present SEI message and all succeeding access units in decoding order until, but excluding, the next access unit that contains a layers not present SEI message or that does not contain any primary coded VCL NAL units with IdrPicFlag equal to 0 (if present), or the end of the bitstream (otherwise) is referred to as the target a