The HOI3 supply system had a lot of problems. It was hard to understand how it worked, and it was hard to know what to do to solve supply problems for the player because it was usually due to missing something some time ago. We even made a separate Arcade Mode for supply which of course nobody used (what self respecting player would pick something called "Arcade Mode"?).
After a couple of glasses of 16yo Lagavulin and some deep thoughts the following problems needed to be solved:
It must at any point be possible to look at an area and see how many troops can work there without problems. Also for naval invasions.
It must be possible to see exactly what the bottleneck in your supplies are and give the player possible actions to fix this that are clear.
The supply system itself must not have long complicated flow networks where cause and effect are hidden by time.
The supply system must not collapse if a capital is taken which was a big problem in HOI3 and both unrealistic and not fun.
Holding out in cities should be possible even when cut off from the rest of your force.
Resistance must be able to hurt your supply lines without being a Whac-A-Mole problem.
Supply issues need to get gradually worse for a unit rather than feel binary like in HOI3.
Scary list? Sure is, but I think we managed to solve all of it. So how does it work then?
The world is separated up in Supply Areas, made up of provinces. They are purely for gameplay and generally follow terrain types and such.
Each area can tell you at a glance exactly how many divisions it can support and how much you are taking up.
If you hover your mouse over an area it will show you an arrow tracing the path supply takes, and indicate what is limiting it. Areas also have quick buttons for helping solve problems right there (improve naval base level or infrastructure etc).
The game will show alerts from areas with supply problems to notify player (super useful when you are, say, Britain and spread over many theaters).
The supply level of an area are decided by the following things:
A local scripted value. Think of it as base infrastructure that can't really be destroyed (will be basically 0 in mountains and deserts and pretty high in densely populated Europe).
Any big cities/victory points will increase it. So holding out in these is possible.
Local resistance movement activity in occupied territory disrupting things for you (we'll have a separate diary on these guys)
Incoming supplies from neighbor area. We trace back to capital, or if capital is cut off the next best area. The supply you get is limited by the lowest infrastructure on your route (also possibly sabotaged by resistance), including your own infrastructure level. So for a player what you need to care about is what the bottleneck is, because that is what is going to affect how troops on the front fare. There is also some guaranteed spillover from neighbors to soften the transitions between bad and good areas (simulating that even if decent railway lines stop at a point it's feasible to transport some distance with trucks or horses etc).
If we are cut off from home area, say fighting in Africa, or on an island, supply will travel overseas using convoys and be limited by the size of the ports receiving it. So making sure convoys are not sunk and bases are able to sustain you is important before doing any overseas activities.
Put transport planes on a mission to drop supplies. Useful for cut off troops (this is still WIP so can't show it yet).
It also worth noting that supply areas will change size if they are being fought over so actual levels will depend on how much you control.
When a unit finds itself out of supply it has a short period of time where they can live off their own supplies, after that their situation will gradually get worse up to about 30 days when things get very bad. Being out of supply makes you lose organization, move slower, not fight as effectively and take a lot more attrition. Veteran players of HOI know that the best way to beat the enemy is to cut off their supply, encircle them and then destroy them, and this remains true in HOI4.
So how can a player improve their supply situation?
You start by finding the bottleneck.
Improve infrastructure to allow more supply into the area.
If linked by sea, make sure to escort convoys and protect them from raiders.
Build bigger naval bases to allow better throughput.
Deal with local resistance.
Research and attach Supply Companies to your division templates to help them manage.
Simply withdraw some troops from an area.
So, what is supply exactly?
In HOI3 supplies was something you produced and stockpiled, then fed into a flow network towards units. In HOI4 the only thing you can stockpile is equipment so this is what you do. Moving, training, fighting, being in bad weather or in particular in bad supply means equipment breaks down and this equipment needs to be shipped. The worse a supply situation is the longer it will take to send equipment and the more attrition you will take. So instead of a flow network we have a system being limited by bottlenecks.
So this means that the abstract "supply" of HOI3 is now instead requests for specific equipment instead which fits a lot better in with HOI4's equipment and production focus. This also means that there is no separate fuel need as such in the game, this instead is included in production of replacement equipment which need Oil (all tanks, trucks etc). Before everyone chokes on their friday beer give this some thought. Being able to stockpile fuel generally leads to the same problems as all other kinds of stockpiling when it comes to hindsight, so by wrapping it into the actual production of equipment requests to units (also nobody would request a tank without diesel to run it, and if they did it wouldn't really be usable as a tank) everything clicks into place and player doesn't have to micro manage all movement, airplane rebasing etc to try to avoid fuel waste and focus on making sure they have access to a strategic Oil resource and replacement equipment and a clear path for units to be supplied.