Choosing auto-negotiation will allow the network card to connect to the network at the fastest possible speed available on the network. If the card has difficulty in negotiating a speed with the network switch, it will choose 10 Half-Duplex.
Note: Connecting the devices mentioned in this knowledgebase to gigabit/1000mbps network switches may still work. Some switches may support 1000/100mbps and will auto-negotiatiate. They may also the user to specifically force the port to 100mbps but are at least capable of doing so. Other switches support 10/100mbps modules/add-ons for connectivity.