Communities normally have heterogeneous undergrowth.
In the Braun-Blanquet floristic classification approach, plant community types are conceived as units recognized by their total floristic composition. Some species within the community are better indicators of a particular community than other species. Diagnostic species are used to organize communities into a hierarchical classification of which the association is the basic unit. In 1935 the International Botanical Congress agreed to use the term association exclusively as defined by the Zurich-Montpelier School as units recognized mainly by characteristic and differential species. There are also numerical classification approaches that are based on floristic composition and species abundance.