The current Secure File Transfer Protocol (SFTP) does not transfer any information about the files to be transferred, only the file contents as a byte stream. This is sufficient for Unix-type files if the sender and receiver use the same CCS.
With MVS datasets, Tectia needs more information: which transfer format to use, what code sets are involved, and what the file characteristics are. Tectia introduces some extensions to SFTP and the information can be relayed by using the Site commands of scpg3 and sftpg3.
For more information on MVS file transfers, see Tectia Server for IBM z/OS User Manual.