If you do not know what you are connecting to, use “Auto”:
The 10/100/1000 Mbps ports operate in either half- or full-duplex mode when they are set to 10 or 100 Mbps and operate only in full-duplex mode when it is set to 1000 Mbps (1 Gbps)
All fiber-optic ports, such as 1000BASE-SX ports, operate only at one preset speed and are always full-duplex
When connecting to known devices such as servers, dedicated workstations, or network devices, a best practice is to manually set the speed and duplex settings