If I have a credit balance of X units then - presumably - I can use them to pay for electricity to heat my house.

But what happens if all of the physical output of the pool has already been claimed by others?

What happens if speculators see a particularly good hedging opportunity and try to buy up as many units as they can?

How does this work if you have tens or hundreds (or thousands) of different units in a market with tens or hundreds (or thousands) of commodities?

Are they interconvertible? How are exchange rates set?