Ok, so there are a few more concepts than that to get your head around. (there is also some work to make sure it is all working as it should and stat balancing.)
Essentially there are mutiple kinds of units, each with strengths and weaknesses.
For example three category types of unit:
Security
Operatives
Military
These function in a paper rock scissors balance system each one gets a bonus against the one below it and a negative from the one above it.
Operatives are almost always invisible except against Security. They will also get withdraw chance probably for hit and run attacks.
Security can see Operatives, they will usually get a bonus to city defence.
Military are just generally stronger and will be the conqueror units.
We then also have different Combat types:
Melee
Ranged
Armour
Etc.
Melee never gets phased out (Just like in Fallout you can go melee all the way to the end). They generally have the highest strength for their class/era units.
Ranged are not as strong as Melee, but they can do ranged attacks with no risk to themselves, range attacks usually cannot kill out right.
(Unless you have promos or a specialist unit like Snipers. The promos may not be functional at the moment, I don't know if dotter ever added them, or if they got lost in the rebuild by archid as he did not port over all of the TLO tags I think...)
Armour is generally better than everything, as it is much tougher.
Siege can bombard cities. (RPG, Mortar, etc.)
Then you have some specialised weapons that work best in certain situations.
EMP(Anti-Robot), Laser (anti-armour except metal armour units), Plasma (anti-armour), Machine/Miniguns (Multiple unit stack damages), Sniper(long range kill shots, vulnerable to melee attacks), Vertibirds (fast, strong versus melee, vulnerable to anti air) and so on.
Different Armours and Vehicles give different benefits (Just like Fallout).
You have the units like slavers that can capture slaves from combat if in a stack and with a slavery type civic active.
So these units then string out in chains.
A Military Chain going through different weapons, armours, vehicles
Security/City Defense Chains
Ops Chain
As well as the various specialised weapons chains like sniper and siege.
Also the more advanced a unit is the harder it will be to build (requiring a higher total pop. in a city) so if you made an entirely vertibird or power armour army, you may find yourself out numbered and out gunned by a horde of axe and rifle wielding maniacs!
As for the city capture, every unit that is supposed to be able (specialist units like snipers shouldn't for example) will have an upgrade unit for it, so as long as you have the cash you can upgrade them to city capture versions.
Eventually these will have better names and a strength increase and a graphic multiplier, essentially representing something of a shift from a single person/small squad to a large squad/small battalion representing an increse in manpower needed to take and hold a town.
We also have access to race types as well now that can have +/- against each other and specific units. So Robot, Human. Mutant, Etc.
Eventually all this stuff will be properly fleshed out and pediaed, it is just one of the many jobs that needs doing Fully!