In vanilla Gates of Hell, Doctrines represent the varying types of military formations that saw action during WW2. After the war, the lessons learned from the widely varying types of battles that raged across the world heavily influenced military planning as the countries of the world sought to make their armed forces more effective.
Therefore, in Hotmod, the haphazard and disorganized Doctrine system has been replaced with a more purpose-designed, streamlined "Formation" system (name not finalized). It represents militaries streamlining their organization, and standardizing weapons and equipment for use in different types of organization. Off-hand, examples include Soviet VDV Airborne units and US Armored Cavalry.
As in the vanilla game, selecting a Formation will give you access to a selection of unique units available only to that Formation type. Here is a list of different formations and some info about the kinds of units they'll offer:
Air Cavalry Brigade: True to the name, Air Cav Formations will have access to a number of helicopter-mounted infantry options including Air Cav Rifle and Weapons squads (with M16s) as well as Airborne Ranger detachments riding to battle in UH-1Ds. They will also have access to a number of helicopters, from the OH-6 Cayuse to rocket and ATGM-armed Hueys, along with the AH-1G Cobra. And, of course, the M551 Sheridan light tank.
Armored Cavalry: ACAV will have access to upgunned and uparmoured APC options for mounted squads, as well as special heavy assault vehicles like the M132 flame tank (based on the M113) as well as the M114A1E2 Recon Vehicle, an ACAV variant of the M551 Sheridan, as well as heavy mortar carriers and M109s.
USMC: The United States Marine Corps will have access to the enormous LVTP-5 transport capable of mounting an entire platoon of Marines (as many as 35!) as well as its assault variant the LTVH-6 armed with a 105mm howitzer. For infantry, Marine Rifle squads are complemented by US Navy SEAL detachments, supported by the M50 Ontos, M67 Zippo flame tank, and America's last heavy tank, the M103.
Armor Brigade: Rounding off the US Formations is the standard Armor or Tank detachment, focusing primarily on tracked, armoured fighting vehicles, such as the M60A2, M728 combat engineer vehicle, self-propelled artillery and the M163 VADS.
Motostrelki: Soviet Motor-Rifle forces will focus on infantry and light vehicles, such as GRU Spetznaz sabotage teams and Guards assault squads, along with BRDM-based ATGM vehicles, heavy anti-tank and medium artillery pieces, and wheeled rocket artillery launchers.
Red Banner: This elite Guards doctrine will have the latest in 1960s Soviet military tech: the BMP-1 IFV, ZSU-23-4 Shilka SPAAG, and T-64 main battle tank, supported by Guards and GRU Spetsnaz infantry squads.
Independant (Heavy) Tank Brigade: Sporting more traditional AFVs like the TO-55 flame tank, T-10M heavy tank and the aging ISU-152M heavy assault gun, backed up by mechanized squads in tracked BTR-50s and a tracked rocket artillery vehicle, the BM-24T.
VDV: Like Air Cav, VDV will have access to a variety of helicopter solutions to battlefield problems in the form of airmobile squads and a variety of attack helicopter options, as well as a suite of Soviet combat vehicles - the ASU-57 and ASU-85 assault guns and the BMD infantry fighting vehicle.
VMF: VMF Naval Infantry will be equipped with an assortment of amphibious capable vehicles, primarily BTR-50P variants with mounted weapons on top, as well as Spetznaz Assault Pioneer Teams.
Rocket & Artillery: Lastly, the USSR will have access to a dedicated artillery detachment offering a variety of tube artillery (152mm and 203mm) as well as truck-mounted heavy rockets like the BM-24 and the then-new BM-21 Grad, along with a host of heavy call-in supports.
So that does it for the two primary armies in Hotmod. As more nations are added, they too will receive Formations, but due to their smaller sizes and more sporadic information, minor factions will not recieve as many as the US and USSR. For example, Great Britain will probably only have Para Brigade and Armoured formations.