What data is NORMAL_MAP expecting?

Godot Version

4.2.1

Question

I’m wondering exactly what sort of data NORMAL_MAP is expecting. It seems redundant to have this in addition to NORMAL and I was wondering if this meant that it accepted different data - ie, the raw rgb values stored in the normal map texture itself rather than requiring the shader to transform them into NORMALs. If my shader does calculate the NORMAL, can I skip using NORMAL_MAP?

I also have a number of normal maps from third parties that only include the red and green channels. While you can calculate the blue channel based on this, I was wondering if it was necessary or if NORMAL_MAP already did this for you?