Official Catan rules

Use the hex-shaped information helper to reference vertices and edges (collectively referred to as nodes).

Green letters refer to the hex vertices where you place cities and settlements. Red letter refer to edges where roads are built. Note that there are potential multiple ways to refer to a node.

For instance, in the image below, the left-most red settlement could have been placed by indicating 13b, 17a, or 14e.

Similarly, the road attached to that settlement could have been place by indication either 17a or 14d