JEITA CP-3451 - 96 - Figure 49 Structure of User Comment Chunk As seen in Figure 49, the chunk data area storing the user comment starts with an 8-byte character code area, where  the  character code is  given.  The  rest  of  the chunk  data area  is  the  user  comment  area,  used  for  writing comments.  The  currently  defined  character  codes  are  shown  in  Table  31.  A  registration  system  is  used  for character codes to avoid duplication. When a character code is registered, a standard document is indicated in the reference column to indicate the character format specification. If a character code is used for which there is no clear specification like Shift-JIS in Japan, Undefined is designated.   The  character  code  is written  from  the start  of  the  character  code  area,  and  the  unused  portion  is  padded  with NULL ('00.H'). In order to match the format of the actual comments in the user comment part to that of the Exif image file UserComment tag, NULL termination is not used even if the comment is written in ASCII text. In accord with the chunk specification, however, a padding byte ('00.H') is used if the chunk data is not an even number of bytes.   Table 31 Character Codes and Character Code Designation Character Code Code Designation (8 Bytes) Reference ASCII 41.H, 53.H, 43.H, 49.H, 49.H, 00.H, 00.H, 00.H ITU-T T.50 IA5^viii JIS 4A.H, 49.H, 53.H, 00.H, 00.H, 00.H, 00.H, 00.H JIS X0208-1990^ix Unicode 55.H, 4E.H, 49.H, 43.H, 4F.H, 44.H, 45.H, 00.H Unicode Standard ^x Undefined 00.H, 00.H, 00.H, 00.H, 00.H, 00.H, 00.H, 00.H Undefined [Notes and Additional Information] The above chunks incorporated in exif list are used with a specific form type ('WAVE') and are therefore written in all lower-case letters. Only the ever chunk is mandatory; the others are optional. If an Exif image file related to an audio file exists, the erel chunk is also mandatory.   When the exif list is read by a camera, information that cannot be recognized may be encountered, such as the emnt chunk or an Undefined string in the eucm chunk. In that case the chunk is to be skipped. An Exif reader and writer shall conform to the stipulations for an ordinary LIST chunk.   A general-purpose RIFF Form Reader will ignore the exif list. A dedicated Exif reader is required for reading the exif list. Figure 50 shows the exif list structure and a coding example. The ever chunk is mandatory and has a fixed length (chunk data is 4 bytes). The erel chunk data is 13 bytes (odd), so a padding byte is added. The erel chunk is also fixed length.