A Classification is an arrangement of taxon in a tree structure. A tree structure arranges taxa in a parent/child relationship. The tree has elements that hold the position of a Taxon Concept, and it's metadata in an arrangement of taxon that we refer to generically as a tree.

A tree changes over time as our understanding of the taxa changes, so a tree can have many versions. Versions of a tree are Publications that can be referenced or cited. You can cite a published tree (version) using an identifier for the tree version or using an identifier for a tree element. This will return the element and then entire tree in the context of the published tree. Each version of a tree is immutable, i.e it doesn't change. You can cite a tree element with confidence that it will not change over time, while being able to trace the history all the way to the current placement.