Record Descriptor Words (RDWs) are the first four bytes at the start of a each record in a variable length data set that tell the reading program the actual length of the current record. When transferring a variable length MVS data set, you can have FTP transmit the RDWs or not include them in the transfer.
This setting is available both when configuring an FTP client and an FTP server. It is applicable to the client when transferring files from the client's system, for example with a PUT. It is applicable to the server when transferring files from the server's system, for example with a GET.
It is very doubtful you will want to transfer the RDWs, because it will confuse most all FTP clients and servers. Most FTP clients or servers receiving the RDWs interpret them as an additional four bytes of data and write the RDW's four bytes into the file as data. The z/OS FTP client and server do not expect the first four bytes of a record to be RDWs and do not interpret them in any special way. Therefore, a z/OS FTP server or client receiving the RDWs will write the RDWs as an additional four bytes of data, thus resulting in an invalid data transfer. You only want to transfer the RDWs if you know the receiving client or server is designed to expect the first four bytes of a record to be the RDW.
If you select "Retain and transfer the RDWs", it is recommended you transfer the file in binary mode to avoid potential translation problems.