The checklist used for pan-species listing is based on the UK Species Inventory (UKSI), which is the standard reference for names and taxonomy for all species of flora and fauna in the UK, and is maintained by the Natural History Museum.
The PSL checklist uses UKSI English and scientific species names but differs in using fewer taxonomic groupings that are preferred for listing, e.g. combining the UKSI "informal groups" of marine mammal and terrestrial mammal into a single Mammals, and including horsetail, fern, ginkgo, quillwort, clubmoss, and conifer, with flowering plant, into Vascular Plants. It also removes many exotic species that UKSI includes but are not deemed appropriate for listing, e.g. escapes that are clearly not established in UK.