An FTP transformation between record-structure and file- structure SHOULD be invertible, to the extent possible while making the result useful on the target host.
RFC-959 required strict invertibility between record- structure and file-structure, but in practice, efficiency and convenience often preclude it. Therefore, the requirement is being relaxed. There are two different objectives for transferring a file: processing it on the target host, or just storage. For storage, strict invertibility is important. For processing, the file created on the target host needs to be in the format expected by application programs on that host.
As an example of the conflict, imagine a record- oriented operating system that requires some data files to have exactly 80 bytes in each record. While STORing a file on such a host, an FTP Server must be able to pad each line or record to 80 bytes; a later retrieval of such a file cannot be strictly invertible.