The SEEA-2003 distinguishes between four types of physical flows: ecosystem inputs, natural resources, products and residuals. Ecosystem inputs and natural resources constitute flows from the environment to the economy. The boundary between them is not clear cut. Products are classified according to CPC and are flows within the economy. Residuals are not clearly defined as they seem to cover flows within the economy and from the environment to the economy. Furthermore, the definition of waste covering both products (with positive value) and residuals (with zero and negative value) is not clear cut. A number of questions need to be addressed, in particular: Should the distinction between ecosystem inputs and natural resources be maintained? Should the definition of residuals and waste -products and residual- be revisited and clarified? Which classification(s) should be used for waste in the SEEA tables (i.e. CPC/HS or EWC Stat)? How should a correspondence be developed between the two? Should the classification of physical flows hold for any type of measurement units used in the various accounts (e.g. water and energy accounts as well as MFA)?