Following is a quote from the developers:
So let’s start with the basics, you can have two types of divisions, reserves and regular. Reserve troops are built cheaper and cost less to maintain but have a strength cap.
For those of you who can remember a distant time back in August 2008 we
announced a game called Hearts of Iron 3. One of the earliest features
we announced was the ability to have reserve divisions and
mobilisation. We thought it was cool then and we still think it’s cool
now.
So let’s start with the basics, you can have two types of divisions,
reserves and regular. Reserve troops are built cheaper and cost less to
maintain but have a strength cap. When you mobilise the strength cap is
lifted and the unit can be reinforced back up to full strength, and
behave just like regular units. Now this of course allows you to catch
countries by surprise, true Blitzkrieg style.
So you are now asking yourself, why in my right mind would I want to
build reserve divisions? The answer to this comes in part from our long
running quest with what to do about those rather silly sized armies you
can see countries like Costa Rica have in the late game in Hearts of
Iron 2. Now the basic system of Hearts of Iron 2 remains intact, you
need manpower to build troops and units need supplies, this limits the
size of your army, but sadly this doesn’t limit your peacetime army
enough. Well we mentioned in passing that divisions require
reinforcement every month to maintain their strength, people don’t sign
up forever, steadily men (and no doubt some women too) leave the armed
forces (the amount is based on your mobilisation law). This puts a
maximum manpower limit on the size of your army, if you don’t have the
people you can’t get a bigger army. It is easier to increase your
mobilisation law when at war or under a lot of threat from neighbours.
However there is a second constraint: Your economy. Maintaining large
peacetime armies carries a high economic cost. Now Hearts of Iron 3 is
not a focused economic simulator so in that regard we were always going
to seek a high level abstraction here, but we still wanted it to have a
cost. The effect we want to set up is the larger your armed forces are
the greater the drain on your economy is, and thus the less resources
you have available for further expansion. So what we have done is
changed how consumer goods are calculated. Instead of it being a fixed
percentage of your IC it is now based on the size of your army. Every
brigade, ship and air wing pushes up your consumer goods demand,
modified by laws and ministers, this means that as your army gets
bigger an increasing proportion of your resources will be sucked into
maintaining it leaving less resources for expansion. When you go to war
your consumer goods demand falls, you can pass laws increasing your
manpower and reducing your loss rate allowing you to increase your
armed forces again.
So where do reserve divisions fit into this? Well firstly they have
lower manpower allowing you to maintain more of them with your
peacetime manpower and secondly non-mobilised reserves cost less
consumer goods than regular troops. This allows you to maintain a
larger peacetime army than you otherwise would have been able to do.
However you do have the minor problem than your evil neighbours might
just overrun you and your patiently built up reserves before they get a
chance to mobilise. Strategy can be quite evil that way, there is no
one right choice.
And since Romania was requested, here is a screenshot of Romania building reserves...
And because maps are cool... here's a view of zoomed out map with alot of counters.
