Substract is a liquid that is mined and then stored and capsule form.
Most Hewns can store mined Substract inside them or on their bodies which means that other players are often not aware of how much Substract is being carried by your Hewn. (This gives players good bluffing opportunities!)
Substract can be used to give your Hewn a number of amazing super abilities:
As Victory Points to win the game. (The more you have the more victory points you get!)
As a currency to buy more Hewns and technologies.
To get extra action points for your Hewns.
To attack other Hewns (like a sort of ammunition.)
To defend your Hewns from attacks.