Use the Logic Op node to generate a clip whose luma (brightness) values are calculated according to the luma values of two source clips. For example, you can add, subtract, or multiply the luma values of corresponding pixels on two clips to create the result clip.
This node functions in the same way as the Logic Ops command in the Processing menu. See Using Logical Operations.
You can add a Logic Op node to any part of the pipeline except to the blend node pipes. This is because the nodes in the CBlend and MBlend pipes must have a single input only, and the Logic Op node always requires two input sources.