The header length must be 4. This type is used to transmit the descriptive information for the file as a whole. This type includes an additional header field for paged files with page level access control information. The header length must be 5. Further header fields may be used to supply per page control information, for example, per page access control.
All fields are one logical byte in length.
The logical byte size is specified by the TYPE command. See Appendix I for further details and a specific case at the page structure. Conversely, FTP implementations must return a file identical to the original if the parameters used to store and retrieve a file are the same. An Internet Encyclopedia 3.1.2.