The Interfaces menu contains an entry for assigning interfaces, and entries for each currently assigned interface. Assigned interfaces will show their configured names, or the standard names if they have not been changed (e.g. WAN, LAN, OPTx)

(assign):Assign interfaces to logical roles (e.g. WAN, LAN, OPT), and create/maintain VLANs and other types of virtual interfaces. See Interface Configuration, Interface Types and Configuration, and Virtual LANs (VLANs).
WAN:Configure the WAN interface. See Interface Configuration.
LAN:Configure the LAN interface. See Interface Configuration.
OPTx:Configure any additional optional interfaces. See Interface Configuration.