x1 is a species of genus x2, family x3, etc.; [open-ended tree-structure categorization].
Also subspecies, order, phylum; (places do not correspond to specific levels in the hierarchy; rather, x1 is at a "lower" or "bushier" part of the tree than x2, x2 is "lower" than x3, etc.; skipping a place thus means that there is one or more known-and-unspecified levels of hierarchy between the two); not limited to Linnean animal/plant taxonomy. See also klesi, lanzu.