Guide
Dry pack vs self-levelling compound
Both make a floor ready for tile or wood, but they work in opposite ways. Self-levelling compound is poured and finds its own level. Dry pack is packed by hand and goes exactly where it is put, flat or sloped.
Self-levelling compound
A cement-based powder mixed to a liquid and poured. It flows out and settles flat, so it is quick on large, open floors that are only a little out.
It is made for thin corrections, from a feather edge up to the thickness the product allows. It needs a primed, clean base and sealed edges so it does not run away, and it cannot make a slope: it always wants to be level.
Dry pack
Sand and cement mixed stiff and packed by hand to screed guides. It can be built from about 1¼ inches up to several inches thick in one go, it can slope in any direction, and it can step up to meet another floor.
It takes more labour per square foot than pouring, but the materials are cheap, and on a thick fill it often costs less than the bags of compound the same fill would take.
Which one your floor needs
It must slope (a shower, a balcony, a terrace, around a floor drain): dry pack. Self-leveller cannot slope.
It is out by more than the self-leveller allows, or it needs to come up to meet another floor: dry pack, or dry pack first and a thin self-leveller on top.
It is a large open floor that is only slightly uneven: self-leveller is usually faster.
It is going over in-floor heating: either can work. A dry pack screed is common where the build-up is thick or the floor also has to step or slope.
Questions
Can self-levelling compound be used in a shower?
Not to make the slope. It levels itself, and a shower floor has to slope to the drain. The sloped bed is dry pack.
Can large-format tile go on dry pack?
Yes. A dry pack bed floated flat is a common base for large tile and stone, which need a flatter floor than most old slabs or subfloors give.