This Technical Report gives details of a general scheme for the classification of gas appliances according to the method of supplying combustion air and of evacuating the products of combustion. This scheme refers to gas appliances that are intended to be installed within buildings and/or to gas appliances intended ) for installation in a partially protected place external to a building. This Technical Report is a guide for the harmonization of product standards and for the common understanding of the types of gas appliances. This TR is not intended to be used as an installation standard or as a product standard, nor as a reference for market surveillance.