Supply Reserves determines how well military buldings support your forces and garrisons. If Supply Reserves is negative your armies will consume additional supply. Otherwise you will receive a small bonus to unit recruitment cost and garrison buildings cost.

It is optional feature and you can activate it in MCM. After this a new lines will appear in treasury tooltip.


The main source of Supply Reserves if the military buildings(red and blue)* reserves from building depends on two factors: settlement level that requires for bulding and region climate:

Suitable climate : from +1 to +3
Unsuitable climate : from +1 to +2
Uninhabitable climate: +1

* - Buildings in Balck Arks Vampire coast ships and Ogre Camps don't count

Any garrison building reduces the Supply Reserves by 1. You can disable it if you select the easy preset in MCT

Each city reduces the Supply Reserves. Small settlements reduce it by 1, but province capitels on level 4 and 5 reduse it futher depends on climate in region

Settlements with suitable climate
Level 1-3: -1
Level 4: -3
Level 5: -5

Settlements with unsuitable climate
Level 1-3: -1
Level 4: -2
Level 5: -3

Settlements with uninhabitable climate
Level 1-5: -1

(it looks strange but otherwise player would never upgrade a settlements in bad climate above level 3)

Each new army reduses the Supply Reserves stronger then previous one
1st army: 0
2nd army: -1
3rd army: -2 (-3 total)
4th army: -3 (-6 total)
5th army: -4 (-10 total)
...etc

So if you have province with 4 cities, 5-tier capital and garrisons in every city, your supply reserves will decrease by 12. To compensate for this, you must construct 4 military buildings (+12 if they are at least tier 3)


Gotrek army do not affect supply reserves

Beastmen and Tomb Kings: don't have the Supply Reserves

Bretonnia:
- minor settlements do not affect supply reserves

Chaos Dwarfs:
- convoys do not affect supply reserves
- outpost settlements do not affect supply reserves

Grand Cathay:
- caravans do not affect supply reserves

Greenskins:
- waagh armies do not affect supply reserves

Khorne:
- blood host armies have their own reserves counter

High Elves:
- all ports increase supply reserves by 1

Skaven:
- vermintide armies do not affect supply reserves for frist 5 turns

Slaanesh:
- disciple armies have their own reserves counter

Warriors of Chaos:
- all ports increase supply reserves by 1

Wood Elves:
- all ports increase supply reserves by 1
- Foraging Groves building increases supply reserves by 2
- Worldroot Pathway, Waystone and Sacrificial Ground buildings increase supply reserves by 1

• I don't recommend enable supply reserves in middle of the campaign. You can but you won't like it. Anyway you can disalble it at any time.
  
• It is ok to have negative supply reserves in early game. 2-3% additional upkeep is nothing