Activity relationship instances are constrained by activity relationship type instances. Exchanges and transformations are refinements of the duality relationship. Transformations represent activity relationships where resources are converted into other resources. Exchanges represent activity relationships where resources are traded with an outside party. Selling chocolates to a customer for cash is a good example of an exchange.
Different rules apply for transformations and exchanges.
For transformations the cost of the inflow economic activity is the sum of the costs of the outflow economic activities. The cost of a chocolate is the sum of all the resources needed to manufacture the chocolate. This is not true for an exchange. The cost of a cash receipt is not the sale amount. The monitoring of imbalances of an exchange relationship is very important.