It falls outside the scope of the formal taxonomic categories. 它不屬于正規植物分類學的分類階元的范疇。
For instance, all birds and reptiles are believed to have descended from a single common ancestor, so this taxonomic grouping is called monophyletic. 例如，所有的鳥類和爬行動物被認為是從一個單一的共同祖先傳下來的，因此這個分類學的分組被稱為單系類群。