This is the basic gist:
There are armor based units and infantry based units.
The armor based units have mainly tanks (either Light, Medium, Heavy, or Tank Destroyer) plus assorted support units.
The infantry based units have mainly normal infantry plus assorted support units. They only ever have light tanks as tank support.
Basically all base units also have motorized (trucks) or mechanized (APCs) versions.
In addition, infantry based units also have "Heavy" types, which include more of the support units, but less infantry.
Armor based units (all included support units are either motorized or mechanized):
Light Armor, Armored(medium tanks), Heavy Armor(heavy or monitor tanks), Destroyer - 100% tanks of the appropriate type.
The above plus:
Infantry: Tanks + Infantry
Assault (NOT Assault Infantry!): Tanks + Infantry + Artillery
Grenadier (NOT Grenadier Infantry!): Tanks + Infantry + Artillery + MG + RPG
All the tank based units have either trucks (if nothing mentioned) or APCs (if mechanized). On foot variants do not exist.
Infantry based units:
Light Infantry: Infantry
Assault Infantry: Infantry + Light Tanks
Siege Infantry: Infantry + Artillery
MG Infantry: Infantry + MG
RPG Infantry: Infantry + RPG
Grenadier Infantry: Infantry + MG + RPG
Storm Infantry: Infantry + Artillery + Light Tanks (Prior to 1.04b3 some have RPGs instead of tanks)
Assault Grenadier Infantry: Infantry + Light Tanks + MG + RPG
Siege Grenadier Infatry: Infantry + Artillery + MG + RPG
Storm Grenadier Infantry: Infantry + Light Tanks + Artillery + MG + RPG
Like I mentioned above, all of those infantry based types also have a heavy variant, which roughly doubles the amount of support troops and halves the amount of normal infantry.
Again, all the infantry based types have foot, motorized and mechanized variants.
[Edit]
Unlocking guide for all infantry formations that are in the huge list below. Please note that you have to have at least one relevant model type developed. So if you want Siege Infantry, you need to have an artillery model available.
Also, this tree is identical to the tree for upgrading & downgrading formations. You have to move along this tree to go from one formation to another. Each tank tree is independent (so no switching from a light armor to an armor formation, or from an infantry formation to a tank formation).
For every formation in that list there are heavy, motorized, mechanized, motorized heavy, and mechanized heavy variants. After unlocking the formation, you automatically unlock the heavy, motorized and mechanized variant. Researching the heavy formation unlocks both the motorized heavy and the mechanized heavy. Researching the motorized also unlocks the motorized heavy (same for mechanized).
All tank formations have the same (but independent) research tree.
All Formations:
Version 2 (now in alphabetical order)
