Now that the mollusk taxon framework extends to all groups, we can add relationships in reference to MolluscaBase. Subgenera are treated as alternate representations there, but at loarie's request those are counted as deviations here. In most cases we don't even need to use subgenera, but with some terrestrial gastropods they are helpful to distinguish species that are otherwise only distinguishable by dissection. Arion in particular is a good example because it has over 50,000 observations. Currently these are the subgenera (and species) we have:
Arion (ater, flagellus, fuligineus, lusitanicus, magnus, rufus, vulgaris)
Carinarion (circumscriptus, fasciatus, silvaticus)
Kobeltia (anthracius, distinctus, fagophilus, hortensis, intermedius, luisae, occultus, owenii, sibiricus, wiktori)
Mesarion (atripunctatus, baeticus, euthymeanus, fuscus, gilvus, hispanicus, iratii, lizarrustii, molinae, nobrei, paularensis, ponsi, subfuscus, transsylvanus, urbiae)

That's in line with MB, with the exception that A. sibiricus is not listed in a subgenus there and they also have A. obesoductus. A. obesoductus and A. intermedius are listed as accepted under Kobeltia and unaccepted under Micrarion, but Micrarion is listed as an alternate representation on its own. So, I think what we have is fine. If people want to add A. obesoductus should we put it under Kobeltia?

Tagging the top IDers to officially record approval for the use of subgenera in Arion. I know these are actively used, but I wanted to make it clear that we want them and want to technically deviate from MB. If this is the case for any other genera, please flag in a similar manner.
@kevin474 @pileated @molissca @huzi0131 @susanhewitt @igor117 @bernhardhausdorf

https://www.inaturalist.org/taxa/47115-Mollusca/taxonomy_details
https://www.molluscabase.org/aphia.php?p=taxdetails&id=818149
https://www.inaturalist.org/flags/552621

Right, Arion is example where subgenera are very useful for practical purposes of identification, because most species cannot be clearly identified without studying genitals, while subgenera are more or less distinct. As far as I know there is no data that would suggest polyphyly in these 4 subgenera, so there is nothing wrong in using them and it's clearly beneficial.

A. sibiricus is Mesarion.