What's the blending equation of THREE.NormalBlending?

I can't find it in the document. Could anyone tell me what's the equation?

I thought it is equal to the following THREE.CustomBlending:

    blending: THREE.CustomBlending,
    blendEquation: THREE.AddEquation,
    blendSrc: THREE.SrcAlphaFactor,
    blendDst: THREE.OneMinusSrcAlphaFactor

But I get a different result when I change the material from NormalBlending to the above CustomBlending.

Here is the link about default CustomBlending.