4. Reserves (yellow)

The primary reserves of an economy are the capital of the system, any stored and concentrated energy resource that can be easily 'freed' to:
1) build more infrastructure (more energy stores), if we use them well or
2) use and consume in order to live (if we don't have a well-designed system)

A Bank is, essentially, a store of resources or energy that can be easily freed and ideally in order to create the productive base to move an economy.

In Nature we have 'energy banks' for example in bees' stores of honey, in the fat of animal's bodies, in carbohydrates of all kinds in plants (potatoes, tree trunks, etc.)

These are some of the human forms we have invented in order to fulfill the same function.