MAP-3100 supports 5 type of operation mode, i.e. Gateway Mode, Relay Mode, Client-Relay Mode, Layer 2 Gateway Mode and Layer 2 Relay Mode.
MAP-3120 supports only 2 types of operation mode: Layer 2 Gateway Mode and Layer 2 Relay Mode. Layer 3, IP related features such as DHCP server, Firewall, Layer 3 Mesh Topology are also not available at this mode.
For more information about those operation modes, please feel free to check the user guide for more.