The web link is to the 1993 version of the code, which is revised periodically. Only a few of the rules in the code are essential to our discussion. The taxonomic system is hierarchical, and every plant has a name within each level of the hierarchy. See the hierarchy account for a description of the hierarchy as applied to tree species. The earliest validly published name of a species is the accepted one.
Later names are illegitimate.
It is only in recent years that botanists have had access to all the world's botanical literature, so some validly published names have been overlooked. For example, Aesculus octandra was long regarded as the valid name for yellow buckeye, until the realization that Aesculus flava was the first valid name, and A. octandra became illegitimate. References to A. octandra must be replaced with A. flava . However, it takes many years to replace the name in tree identification books, which are updated infrequently. As a result, there are many illegitimate names in use.