FIELD: information technology.SUBSTANCE: method comprises the step of decoding a unit type value of network abstraction level NAL for a NAL units of supplemental enhancement information SEI of a bitstream. Wherein the NAL unit type value for a NAL SEI unit indicates whether a NAL unit comprises a prefix SEI NAL unit including a prefix SEI message or a suffix NAL SEI unit that includes a suffix SEI message. Further video bitstream data decoding is carried out based on whether a NAL SEI unit is a prefix NAL SEI unit wherein a prefix NAL SEI unit can not follow the last NAL unit of video coding levels VCL in an access unit in decoding process.EFFECT: increased video data coding and decoding efficiency.14 cl 10 dwg 8 tbl