Where to find it:

Layerpalette / Blendmodes

NodeGraph / Merge Node / Blendmodes

Painting Toolbar / Blendmodes

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Linear Light

Add/Sub

Add/Sub Halve

Subtract

Custom Blendmodes extend Mari's extensive list of Blendmodes

Burns or dodges the colors by decreasing or increasing the brightness, depending on the blend color. 

If the blend color  is lighter than 50% gray, the image is lightened by increasing the brightness. 

If the blend color is darker than 50% gray, the image is darkened by decreasing the brightness.

Example of Linear Light Blend Mode

This Blend mode is similar to a Vivid Light Blendmode which instead of increasing/decreasing the brightness,

will increase/decrease the Contrast.

Example of Add/Sub Blend Mode

This Blendmode works computationally the same as the Add |Sub Blend Mode found in the Substance Suite

oForeground pixels ('Over' on a Merge Node) with a value higher than 0.5 are added to their respective background pixels.

oForeground pixels ('Over' on a Merge Node) with a value lower than 0.5 are subtracted from their respective background pixels.

A + ( ( B - 0.5) * 2.0 )

The result is NOT clamped

Example of Add/Sub Halve Blend Mode

This Blend mode is similar to an Overlay but subtracts a value of 0.5 first before combining

A + (B - 0.5)        

The result is clamped to 0-1.

Compared to the Add | Sub Blendmode this Blendmode tends to favor the Background Texture and generally builds up less contrast,

when applied multiple times. 

Example of a Mask from Curvature Node configured with using Add|Sub Halve vs Add|Sub

A straight subtraction

(A-B)

The result is NOT clamped