A multilateral exchange would involve a third party, for example: Alice gives an apple to Bob who gives an orange to Charles, who gives a pear to Alice.
In the real world, such transactions are spread over time, involve items of different values, and involve many more parties.
Although any accounting framework can be used, there is one approach that fits naturally for multilateral exchange.
When a transaction happens, an entry is made in an accounting journal of a payment, or credit flowing in the opposite direction.
thus a balance of +10 means that not only has the account delivered +10 more value to the other members than it has received, but that in order to complete the exchange it intends to receive +10 more in the future than it delivers.