The physical flow accounts presented in [SEEA] are based around the distinction between natural resources, ecosystem inputs, products and residuals. Natural resources and ecosystem inputs enter the economy; residuals eventually leave it. Economic activity itself is concerned with the production and consumption of goods and services (products). All natural resources and ecosystem inputs absorbed by the economy are converted into products by one means or another. All physical products eventually return to the environment as residuals. It is the process in between, when products circulate within the economy which is of interest for economic accounting since this is the point in the cycle when monetary values can be associated with the flows.