Apart from the relationships between units established by the derivation history or by ensemble sets, further associations of units exist. They must be defined where units are so intimately related that they are usually cited together. This is particularly important to express host-substrate relationships, but it can also be used to express a variety of other ecological or curatorial associations between units.
The association is essentially a many-to-many relationship between units. Instances of the entity Association define pairs of associated units from which the complete association set can be compiled. The attribute association type specifies the relationship, e.g. "living on" (e.g. orchid growing on a tree), "parasite on", "host of", "mutualistic symbiosis", "reobservation" (the same individual has been observed on two different dates), "also collected as" (two collectors assign different numbers to the parts of the same material), and "mycorrhizal association". An association doubtful flag can be used to express insecurity (e.g. a toadstool is collected and presumed to be a mycorrhizal fungus of a nearby observed tree). The direction of the link may be important (e.g. in "parasite of", but not in "reobservation"), in which case the directed flag is set.