I just wrote 500 words explaining the relationship between Taxonomy, Nomenclature and PESI for use in the PESI portal. Here they are:
The process of creating a classification of life is split into two parts. Firstly experts decide which species exist. This process is called taxonomy. Secondly the experts work out what to call the species they recognise. This is called nomenclature.
The relationship between taxonomy and nomenclature is complex. The same species may have been discovered and named more than once by different people. A single species may be found to contain more than one cryptic species. Changes in our understanding of the relationships of species can also result in changes of the names – even if the species themselves do not change.
Nomenclature is governed by a set of rules. For animals these rules are given in the International Commission on Zoological Nomenclature code (ICZN) for plants and fungi they are given by the International Code of Botanical Nomenclature (ICBN). Because the naming of a species is only a matter of applying the rules in these codes (and not a matter of judgement or scientific opinion) nomenclature is objective and could even be automated.
Taxonomy reflects current scientific opinion. Two experts may disagree about whether populations represent separate species or how species should be classified. As our knowledge of biodiversity increases classifications may change reflecting changing opinions through time. Taxonomy is therefore subjective.
Because nomenclature is objective it should be straightforward to build a database of all the published names of organisms along with where they were published and what their voucher specimens are but without any indication as to whether they are the currently accepted names of species. This would make it very easy for scientists to work out the correct name for a species when they define it. Such databases of purely nomenclatural information are called nomenclators. Attempts have been made to build nomenclators for each of the nomenclatural codes. Examples include the International Plant Names Index (IPNI) which attempts to list all names of vascular plants, Index Fungorum to list names of fungi and ZooBank names of animals.
PESI is a taxonomic resource. It is an annotated checklist of European organisms that reflects current scientific opinion. As with other scholarly taxonomic works PESI contains much nomenclatural information for organisms and so is also a valuable source of purely nomenclatural information.
Because no nomenclator is yet complete and there is no single point of contact for all scientific names (plants and animals) there is an initiative to build a Global Names Architecture (GNA) that will unify all these efforts. There are further aspirations to produce a Global Names Usage Bank (GNUB) that will include a register of taxonomic opinion on these names.
PESI therefore has a two way relationship with nomenclators. It uses them as sources for correct nomenclature and it can also provide nomenclatural data for inclusion in them where there are gaps. Going forward PESI will be a part of both the GNA and the GNUB both by contributing data and by benefiting from the contributions of others.