The basic unit of syntaxonomy is the "association", defined by its characteristic combination of plant taxa, habitat features, physiognomy, biogeographical area, role in ecological succession, historical (e.g. history of use by humans) and paleo-biogeographical relationships. The association is sometimes viewed as a concrete phytocoenosis (the plant component of a biocoenosis). Associations with floristic and territorial affinities[clarification needed] can be grouped in larger ecological conceptual units (i.e. syntaxa) called "alliances".