Concepts
Embark
In most scenario an unit need to be adjacent to a city with an harbor to be able to embark.
Paradroop
Paratroopers can only paradrop from a city or an airfield.
Reinforcements
To heal 1 hit point, any unit need "reinforcements", a combination of "personnel" and "materiel". Some unit will use more personnel (Infantry), some more materiel (Tanks), some a bit of both (Light Tanks)
Personnel are generated from your total empire
food production, from specific buildings in cities (see tooltips in construction list), from "We Need You" process in cities, and you receive a portion of what your friendly and allied city-states are producing for themselves.
Materiel are generated from your total empire
production, from specific buildings in cities (see tooltips in construction list), from "Buy War Bond" process in cities, and you also receive a portion of what your friendly and allied city-states are producing for themselves.
"We Need You" is proportional to the city food production, "Buy War Bond" is proportional to the city 
and production. Those two process are important, and quite useful with a bit of micromanagement.
On some maps there are "convoy" units that move automatically on maritime routes under specific conditions and bring you personnel, materiel, gold or new units.
Retreat
When it take enough damage (defending in melee combat) to make it think that 3-4 attacks of this level will kill it, an unit will try to retreat from it's position.
Examples : Infantry at full health (75hp) can take 10 damage from a tank and won't retreat, but at half health (37hp) it will. Introducing morale is planned, but still not done.
Unit will retreat to one of the 3 plots opposing the attack direction, if none are free, the unit will get half more damage (5 in above example), but will always keep at least 1hp.
Retreating from city is more specific, a unit is not killed in a captured city, it will try to escape before it happens, and will loss hp in the process. If there's a free valid plot just around the city, it will go there, else it will be transferred to a nearby city, with a bigger hp penalty.
All routed units will lost moves for the next turn.
Frontline
Units capture enemy territory when moving on his tiles. The territory map is memorized at game start, and you can liberate old territory from friend/allied civs.
Some units don't capture tiles, like special force and artillery.
Buildings
Some buildings require another to be available (ex: Land Factory require a Factory), and some will block the construction of other (ex: a city with a Land Factory can't construct any Air Factory). Units will require specific buildings. Use the mouse over help text in the production screen to know which buildings or units will be made available by the one you're about to build.
See the mod's spreadsheet for more info.
Projects
Projects can be produced in cities with special buildings. New models (example: T-34) projects require a Military Academy, which require Barracks to be constructed. Models evolution (ex: T-34 to T-34/76) require either a Military Academy OR the building needed to construct the unit (a Land Factory in that example)
Projects are unlocked around specific dates, or after units from a specific type have gained enough experience from combat.
There is another kind of project called "Military Operation". Those are in beta testing and only two of them are available ATM, for Germany on the Europe 1939 map. Those projects allow fast deploiement of units in a theatre of operation (example : "Operation Weserübung" will deploy 3 paratroops division (the AI get more) in Norway for Germany. A Military Base is needed to work on such projects.
Embark
In most scenario an unit need to be adjacent to a city with an harbor to be able to embark.
Paradroop
Paratroopers can only paradrop from a city or an airfield.
Reinforcements
To heal 1 hit point, any unit need "reinforcements", a combination of "personnel" and "materiel". Some unit will use more personnel (Infantry), some more materiel (Tanks), some a bit of both (Light Tanks)
Personnel are generated from your total empire
food production, from specific buildings in cities (see tooltips in construction list), from "We Need You" process in cities, and you receive a portion of what your friendly and allied city-states are producing for themselves.Materiel are generated from your total empire
production, from specific buildings in cities (see tooltips in construction list), from "Buy War Bond" process in cities, and you also receive a portion of what your friendly and allied city-states are producing for themselves."We Need You" is proportional to the city
food production, "Buy War Bond" is proportional to the city and production. Those two process are important, and quite useful with a bit of micromanagement.On some maps there are "convoy" units that move automatically on maritime routes under specific conditions and bring you personnel, materiel, gold or new units.
Retreat
When it take enough damage (defending in melee combat) to make it think that 3-4 attacks of this level will kill it, an unit will try to retreat from it's position.
Examples : Infantry at full health (75hp) can take 10 damage from a tank and won't retreat, but at half health (37hp) it will. Introducing morale is planned, but still not done.
Unit will retreat to one of the 3 plots opposing the attack direction, if none are free, the unit will get half more damage (5 in above example), but will always keep at least 1hp.
Retreating from city is more specific, a unit is not killed in a captured city, it will try to escape before it happens, and will loss hp in the process. If there's a free valid plot just around the city, it will go there, else it will be transferred to a nearby city, with a bigger hp penalty.
All routed units will lost moves for the next turn.
Frontline
Units capture enemy territory when moving on his tiles. The territory map is memorized at game start, and you can liberate old territory from friend/allied civs.
Some units don't capture tiles, like special force and artillery.
Buildings
Some buildings require another to be available (ex: Land Factory require a Factory), and some will block the construction of other (ex: a city with a Land Factory can't construct any Air Factory). Units will require specific buildings. Use the mouse over help text in the production screen to know which buildings or units will be made available by the one you're about to build.
See the mod's spreadsheet for more info.
Projects
Projects can be produced in cities with special buildings. New models (example: T-34) projects require a Military Academy, which require Barracks to be constructed. Models evolution (ex: T-34 to T-34/76) require either a Military Academy OR the building needed to construct the unit (a Land Factory in that example)
Projects are unlocked around specific dates, or after units from a specific type have gained enough experience from combat.
There is another kind of project called "Military Operation". Those are in beta testing and only two of them are available ATM, for Germany on the Europe 1939 map. Those projects allow fast deploiement of units in a theatre of operation (example : "Operation Weserübung" will deploy 3 paratroops division (the AI get more) in Norway for Germany. A Military Base is needed to work on such projects.
