Here are 'The Rules' about what you can and can't count on a pan-species list. Many pan-species listers take different approaches, so these are best thought of as guidelines.
Geography: The biogeographical unit of ‘the British Isles’, i.e. Britain, Ireland and the Isle of Man, including the seas around the isles (defined for the UK as the UK Economic Exclusion Zone of 200 nautical miles (370 kms) or midpoint between the UK and any neighbouring country). The Channel Islands also count, even if they are biogeographically part of France.
Taxonomy: All species in the animal, plant, fungus and protist Kingdoms, i.e. everything except Bacteria and viruses. Bacteria are excluded partly because they have not been traditionally covered by naturalists and partly because the very concept of a species is difficult to apply to Bacteria.
Taxonomic level: Only species count as ticks.
Sensory: You must see the species, not just hear it. It doesn’t count if you see it only via television or digital camera. Most pan-species listers count gall-causers and leaf-miners only when they've seen a living occupant but some are happy just to see the diagnostic characters of the gall or leaf-mine without laying eyes on the species itself.
Alive or Dead: Dead wildlife doesn’t count. But there is an exception. Conventionally, many entomologists count species that they have only seen dead, such as species they have caught in lethal trap samples (e.g. pitfall traps, malaise traps). Because some of the leading pan-species listers would be unable to work out their pan-species lists excluding species they've only seen in trap samples, then counting dead species is allowed in this circumstance. Of course, trapping should only be carried out where the results are likely to justify the casualties, e.g. on a site that is scheduled for development.
Free or Captive: It is scraping the barrel to tick things in captivity but most moth-ers are happy to tick moths in pots in fridges. So pan-species listers can tick things that are being temporarily held captive, if they wish. Species in long-term captivity (e.g. zoo or farm animals) or culture (e.g. crops, garden plants) don’t count.
Developmental stage: All developmental stages count. For example, eggs, larvae, nymphs and pupae count just the same as adult insects, if they can be confidently identified. Likewise, plants that are not in flower can be counted (even though not everyone chooses to do so). Even seeds can be counted as long as they are alive, which raises the possibility of adding sea beans, nickar nuts and other marine drift seeds if you can find them and get them to germinate.
Aliens: I think this is the area, more than any other, where attempts to make a simple set of rules that can be applied consistently across all taxonomic groups, are doomed to failure! There is a spectrum from native species that have lived in Britain from before the time that humans started to make their mark on the planet, up to alien species that have just been intercepted on arrival at one of our ports. Although we value the true natives above all others, we find the whole spectrum fascinating. So, the ‘line in the sand’ is drawn to include the majority of aliens as long as they have established, or seem capable of establishing, without deliberate human assistance. You can count any garden plant that has dispersed and established beyond the garden fence. You can count any invertebrate that has established, even if only in highly man-modified environments (e.g. only in warehouses or heated greenhouses). But you can’t count any of the invertebrates that you can occasionally find in your groceries as primary imports but which are unlikely to be able to survive here unaided.
Hybrids: Ordinarily, interspecific hybrids are evolutionary dead-ends with little or no fertility: not countable. But amongst plants at least, there are numerous species which have a hybrid origin, usually formed by polyploid hybrid speciation. Pan-species listers won’t want to count a hybrid unless it can reproduce and persist in the absence of one or both parent species. The wording “one or both” is deliberately chosen: Edible Frogs and various Sorbus species are countable but have a hybrid origin and need reproductive contact with one of the parent species to be able to reproduce themselves. In practice, it’s not always easy to determine whether a hybrid plant is countable or not. Dave Gibbs suggests that as a rule of thumb, any hybrid that is normally known by its own name is a countable one (e.g. Viola x wittrockiana, Garden Pansy) whereas any that is normally known by the combination of both parents is not countable (e.g. Viola riviniana x V. reichenbachiana, the hybrid of Common and Early Dog-violets).
Be your own judge: Most pan-species listers have their own rigid standards about what they can and can’t count. Mark won’t count anything that’s in a pot. Dave won’t count plants unless he’s seen them in flower. Martin won’t count anything unless he’s found it for himself. We only need a set of rules if we’re competing against each other to get the biggest list. But pan-species listing is more about the personal challenge to get a grip on the immense biodiversity of these islands.