This version of Unity is unsupported.

TerrainData.AlphamapTextureName

public static string AlphamapTextureName;

Description

The name for the Terrain alpha map textures.

Use this name when you call CopyActiveRenderTextureToTexture, DirtyTextureRegion, SyncTexture, or TerrainCallbacks.onTextureChanged to identify the Terrain alpha map textures.