From my quick Google, ammonium nitrate gets processed into...

From my quick Google, ammonium nitrate gets processed into prills (little spheres). It's the same process to make the prills whether they are high density or low density. High density prills are used for fertiliser and low density prills are used for explosives (their lower density makes them porous, so they can absorb 4% fuel to make ANFO - ammonium nitrate fuel oil mixture).

so "high density" means fertiliser grade AN in prill form. "Low density" means explosive grade, aka technical grade.

i assume that means the spot price is the fertiliser grade AN price?

urea is a different thing, MNB isn't making that