When to use absorption, and when diffusion?

When to use absorption, and when diffusion? Amongst all the voodoo in studio acoustics, I don’t think there’s anything more voodoo than diffusion. And I don’t mean like with sound crystals. Diffusion does absolutely work. But amongst the stuff that is part of the standard acoustics toolkit, it’s definitely the thing with the most myth surrounding it. To the point where there are people out there who will literally answer with “diffusion!” when you ask them about the one thing you should think...