This is for TerrainComposer1 only, TerrainComposer2 has runtime included.
TerrainComposer can be used in runtime. It has no limitations compared with TerrainComposer in Unity Editor mode. As I got many requests for runtime I deciced to release the documentation package for it. The documentation is not entirely finished yet, but TerrainComposer runtime is used by many users and they all are very pleased with it. This package includes an example Unity script and documentation that explains how it works. I made a new TerrainComposer runtime package with a C# example script included that uses reflections.