How does the fragment shader know what variable to use for the color of a pixel?

Question

I see a lot of different fragment shaders,

#version 130

out vec4 flatColor;

void main(void)
{
    flatColor = vec4(0.0,1.0,0.0,0.5);
}

And th

Answer 1

I'd like to specify this for OpenGLES 3.1 which uses GLSL_ES_3.10 link:

§4.4.2

If there is only a single output [in the fragment shader], the location does not need to be specified, in which case it defaults to zero.