Intermediate nodes are part of the Layer 3 network used for interconnections among the devices operating in a fabric role such as the interconnections between border nodes and edge nodes.
These interconnections are created in the Global Routing Table on the devices and is also known as the underlay network.
https://www.cisco.com/c/en/us/td/docs/solutions/CVD/Campus/cisco-sda-design- guide.html#EdgeNode