Supplemental enhancement information
SEI messages are a special parameter that is specified for H.264 and H.265. SEI message may be used to specify how the frame should be displayed. SEI is predefined to enable mechanisms of display, decoding but is not essential for configuration decoded video frames. Bitstream confirmation is checked by some SEI messages information. SEI messages have special types like Buffering period - provides information for Hypothetical Reference Decoder (HRD), Picture Timing - provides information for the access unit or controls the timing of HRD, Scene information - identifies a set of consecutive pictures like it would be in a video scene, and others messages.
Each message for H.264 is transmitted in an SEI Raw Byte Sequence Payload as a detached NAL (Network Abstraction Layer) Unit.
There are some SEI message variants for H.265 such as prefix SEI message and suffix SEI message. It means that the messages are contained in the prefix SEI NAL Unit or in the suffix SEI NAL Unit which types equal to PREFIX_SEI_NUT or SUFFIX_SEI_NUT respectively. SEI messages can have different payloadType values.
SEI messages contain some fields whose syntax and semantics are defined by the specifications.