That is a neat function above with the volumetric lighting. I was reading through it and realized that part of it could be simplified, and maybe that will help others understand it better.
Mostly these two lines:
float3 S = CustomExpression0(Parameters,p,t) * mu * phfu* CustomExpression2(Parameters,p,lightPos,t);
float3 Sint = (S - S * exp(-mu * dd)) / mu;
starting with the bottom one, if we have (S - S(x)), this can be rewritten as S(1-x). This is a bit easier to read since it implies we are applying a multiplier to an inverted density function. Also the term mu can simply be removed from both lines as it is simply being divided out. So those two lines become:
float3 S = CustomExpression0(Parameters,p,t) * phfu* CustomExpression2(Parameters,p,lightPos,t);
float3 Sint = S(1 - exp(-mu * dd));
btw this shader was a bit slow since it is doing 300 * 30 = 9000 samples of the density function. Something like this could be much faster using distance field shapes to accelerate the tracing.
Also terms like pfhu could simply be factored into your light color as another optimization to avoid more math at each sample.