This is what the function does:

switch (_BIS_fadeInOrOut) do
{
	case 0:
	{
		...
		_BIS_fadeDuration fadeSound 0;
		_BIS_fadeDuration fadeMusic 0;
		_BIS_fadeDuration fadeSpeech 0;
		...
	};
	
	case 1:
	{
		...
		0 fadeSound 0;
		0 fadeSpeech 0;
		...
		_BIS_fadeDuration fadeSound 1;
		_BIS_fadeDuration fadeSpeech 1;
		...
	}
};

In other words, if someone uses the function in "fade out" mode ([0,"BLACK",3,1] spawn BIS_fnc_fadeEffect;), they can't go back to normal using [1,"BLACK",3,1] spawn BIS_fnc_fadeEffect

But at the same time, I don't think it is necessary to use 0 fadeMusic 0; at the start of case 1. (it changes the behavior of this function)
you only need:
_BIS_fadeDuration fadeMusic 1;