World Machine is a very powerful node-based editor that allows for the creation of very complex and realistic landscapes and terrain.  One of the recent features it has is called a splat map, which can also be known as a weight map.  These are black and white masks that match the terrain, and break out areas by height, slope angle and other parameters.  When set up properly in an engine like Unity, it allows different textures masking in a very realistic manner, that follows the height map information closely.  Below is the height map information displayed in World Machine, with colorization done by slope & height masking.

Below is the same file, with a pass at the splat map breakout.  Each channel (R,G,B, A) has a specific slope or height data.  The combined file is set up in Unity, then the terrain textures are fed into each channel of the splat map.