This isn't ioFTPD alone. Your FTP client send those command to know if the FTP server you're connecting to supports resume or not.
Code:
[09:24:24] [R] REST 100
[09:24:24] [R] 350 Restarting at 100. Send STORE or RETRIEVE to initiate transfer.
[09:24:24] [R] REST 0
[09:24:24] [R] 350 Restarting at 0. Send STORE or RETRIEVE to initiate transfer.
If the client receives a positive answer to the REST 100 command, then the server supports it ; otherwise, it doesn't. The REST 0 command is then issued to make sure you won't try to resume a file you haven't started to transfer.