This node provides a modern way to generate and layer bump maps, providing an inexpensive way to add small surface details through shading rather than modelling modeling them.

Traditional options do not allow for order-independent mixing and this node provides for easy mixing of multiple inputs if needed. You can also chain these nodes together for more complex networks using masking to vary bump maps across surfaces while using a single material assignment.

Nodes that supply a resultNG output can connect to this node, examples include PxrTexture and many procedural patterns. This new NG vector type expresses the signal's gradient and is more accurate and efficient to compute than traditional bump map techniques.

...

A simple grayscale (scalar) pattern or texture channel is used to mask where you want this particular layer to take effect. This is especially useful when using the Over mode.

...