What is a "node license"?
Node license (often shortened to "N") is a license that may be used on a node PC only - where the host application (3ds Max, C4D, ...) is running in command line mode, with no GUI - for example when rendering using Corona's Distributed Rendering in 3ds Max and/or Autodesk Backburner, or using Team Render in Cinema 4D. It is the opposite of workstation license.
Each workstation license can be also used as a render node license, so having 2 x [1 workstation + 3 render nodes] licenses means that you can use them on 2 workstations and 6 render nodes, but also on 1 workstation and 7 render nodes.
Note: you do not need a 3ds Max license to render on a node PC. Trial license of 3ds Max can be used for this, even if it's already expired. This is absolutely fine according to the Autodesk licensing: