Use the AUXDATA command to signal the start of a burst of "auxiliary data".
This data is binary data outside of the main file contents.
It is typically used to carry the "resource fork" or "extended attributes" of the file.
Any AUXDATA statement must come after a FILE command and preceed any EOF, ABORT, or QUIT command. AUXDATA will usually also follow a USER command.

The syntax is
  AUXDATA     burst size  

burst size   is the number of bytes (octets) about to be sent

When the server receives a AUXDATA command, it will then consume the next burst size bytes (the next burst size octets) as auxiliary data. After receiving exactly that many octets, it waits for another command from the client.

See also