In Michigan I use Howard Crum's Mosses of the Great Lakes Forest. This is not a global source.
I don't know whether there is a "global" agreement on this. Crum uses the term "Section" while others use the term "Subgenus." I will post the question on Bryonet.
I retract my request for the addition of Sphagnum subgenera. The most practical way to solve this issue will be to ID my observations as Sphagnum and add any tentative subgenus in the description section.
pinging @cathymtp
Is there a global source that covers all the species of sphagnum and to which subgenus each belongs?
You can read more about the curation of additional taxonomic nodes here:
https://www.inaturalist.org/pages/curator+guide#nodes