Series type alters how it’s searching and what it can do with parsed data (a series set to standard won’t import a file that uses a date as the identifier). In this case it doesn’t really matter because standard actually has data, but anime doesn’t because they lack absolute episode numbers (even though they are using an absolute episode number for the release.
Depends if they’re consistent enough in their scheme, Happy Together 3 E512 720p NEXT has a “season” number of sorts but also an absolute-like number. Which differs from Infinity Challenge E543 170819 720p NEXT that just uses an absolute-like number. If all the episodes, despite being in different seasons never conflict (season 2/3/x doesn’t restart at episode 1), then Sonarr could potentially have another series type that converts those episode numbers to absolute episode numbers (or on lookup it just searches by episode number). I’m not sure how that’d work with the anime type being similar, yet different.