It seems to be true for optics, but there are situations where L1 isn't the only layer interacting (if that's what you mean by "priority"). One example of that is CONV still interacting with surrounding particles from any layer 2+. Heat also transfers through layers and onto neighboring particles. But yeah I think I misunderstood what you said earlier, apologies for that.
It feels like the game takes priority of whatever is in L1
Gowate: What I meant is, if you look at them as in this save, there seems to be a difference in behaviour that stems from stacking order. While actually, it doesn't do anything new. GLAS makes rainbow streams of PHOT, and QRTZ creates a diffusion effect. Oh, well.
gatedestroyer: actually, yes exactly. id:3428631, id:3428633
Gowate: Not exactly, although I didn't know that normal QRTZ causes phot to diffuse. However, it clearly shows that stacking order matters.
Type 1 and 3 don't really do anything new. Type one acts the same as just QRTZ and type 3 as GLAS