What protocol can be applied when you want to transfer files between different platforms, such between UNIX systems and Windows servers?

You can use FTP (File Transfer Protocol) for file transfers between such different servers as FTP is platform independent. It is a standard network protocol used to transfer computer files from one host to another host over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and uses separate control and data connections between the client and the server.

FTP = File Transfer Protocol and SFTP = Secure File Transfer Protocol