Depending on your server's capabilities and the duplex mode of the link partner you are attaching to, you can use either full or half duplex.
Half duplex mode will restrict the server to one way communication; it cannot transmit and receive data at the same time. Full duplex allows your server to send and receive data simultaneously. You must match your server's duplex mode to that of your link partner. If you do not, a duplex mismatch will occur.