The different interface types on the USG devices

Depending on the model, the USG has a certain amount of interfaces. On the smaller USGs like the USG60, the interfaces are clearly differentiated with their names.
The LAN is the internal, WAN the external interface, and the OPT can be either.

On bigger models like the USG310, the ports are labelled as "GE1, GE2..." and so on, depending on the number of ports. These can be freely setup to any of the available interface types.
Here are more information about the different types:

• internal is for connecting to a local network. Other corresponding configuration options: DHCP server and DHCP relay. The Zyxel Device automatically adds default SNAT settings for traffic flowing from this interface to an external interface.
  
  
• external is for connecting to an external network (like the Internet). The Zyxel Device automatically adds this interface to the default WAN trunk.
  
  
• For general, the rest of the screen’s options do not automatically adjust, and you must manually configure a policy route to add routing and SNAT settings for the interface.