In order to stabilize the defining concepts of the higher rank syntaxonomic units such as class or order, the criteria of floristic content and unity of origin are enunciated. This is done with the aim of preventing the fragmentation of the large classes and the subsequent typological inflation. For orders, the criterion of specific floristic content is discussed, with orders that have been described to encompass seral secondary forests or the separation of forest vegetation from that which is dominated by shrubs rejected, due to their weak floristic characterization. These criteria have been applied to two forest vegetation classes: the European temperate (Querco-Fagetea) and the Mediterranean (Quercetea ilicis). For the first, it is argued in favor of maintaining a single class for all temperate deciduous forests in Europe instead of dividing them into four. Within this single class five orders are distinguished: Fagetalia, Quercetalia roboris, Quercetalia pubescenti-petraeae, Alno-Fraxinetalia and Populetalia albae, rejecting the orders that have been proposed for secondary forests because they have few characteristic taxa. For the sclerophyllous and macchia forests of Mediterranean Europe, the Quercetea ilicis class can be split into two or three geographical orders, rejecting the Pistacio-Rhamnetalia alaterni as a shrubby physiognomic unit. Taxonomic reference: Castroviejo S (coord. gen.) (1986–2012) Flora iberica 1–8, 10–15, 17–18, 21. Real Jardín Botánico, CSIC, Madrid, ES. Syntaxonomic reference: Mucina et al. (2016).