1 (plant formation) In vegetation description and analysis, a classificatory unit which usually implies a distinctive physiognomy rather than a distinctive species composition. In detail, the various phytosociological traditions ascribe slightly different meaning and hierarchical status to the term. The early European schemes grouped associations into alliances and then formation groups (e.g. sclerophyllous scrub), and then formation classes (e.g. sclerophyllous woodland and scrub). The formation classes are roughly equivalent to the major world biomes.