This is an extract from GIF89a Specification.

26. Application Extension.

      a. Description. The Application Extension contains application-specific
      information; it conforms with the extension block syntax, as described
      below, and its block label is 0xFF.

      b. Required Version.  89a.

      c. Syntax.

      7 6 5 4 3 2 1 0        Field Name                    Type
     +---------------+
  0  |               |       Extension Introducer          Byte
     +---------------+
  1  |               |       Extension Label               Byte
     +---------------+

     +---------------+
  0  |               |       Block Size                    Byte
     +---------------+
  1  |               |
     +-             -+
  2  |               |
     +-             -+
  3  |               |       Application Identifier        8 Bytes
     +-             -+
  4  |               |
     +-             -+
  5  |               |
     +-             -+
  6  |               |
     +-             -+
  7  |               |
     +-             -+
  8  |               |
     +---------------+
  9  |               |
     +-             -+
 10  |               |       Appl. Authentication Code     3 Bytes
     +-             -+
 11  |               |
     +---------------+

     +===============+
     |               |
     |               |       Application Data              Data Sub-blocks
     |               |
     |               |
     +===============+

     +---------------+
  0  |               |       Block Terminator              Byte
     +---------------+

            i) Extension Introducer - Defines this block as an extension. This
            field contains the fixed value 0x21.

            ii) Application Extension Label - Identifies the block as an
            Application Extension. This field contains the fixed value 0xFF.

            iii) Block Size - Number of bytes in this extension block,
            following the Block Size field, up to but not including the
            beginning of the Application Data. This field contains the fixed
            value 11.

            iv) Application Identifier - Sequence of eight printable ASCII
            characters used to identify the application owning the Application
            Extension.

            v) Application Authentication Code - Sequence of three bytes used
            to authenticate the Application Identifier. An Application program
            may use an algorithm to compute a binary code that uniquely
            identifies it as the application owning the Application Extension.

      d. Extensions and Scope. This block does not have scope. This block
      cannot be modified by any extension.

      e. Recommendation. None.