Parallel mode is a special uploading optimization in edtFTPj/PRO.
Typically, to write a chunk of data to the server requires sending an FXP_WRITE message, which is acknowledged by the server, and then another FXP_WRITE is sent with another data chunk.
But most servers can cope with multiple FXP_WRITE messages being sent without waiting for acknowledgements. The server processes them and sends the acknowledgements asynchronously. This is often considerably faster.
Parallel mode is enabled by default.
This isn't available for downloading from the server - it only applies for upload.