Hi @iptz1,
Good questions! The _A labels are directly from GTDB (see here for why). The _<number> is used to represent a distinct node in the phylogeny. In this case, "g__Blautia_A" is supported by more than one node, so we have to differentiate them to ensure the taxonomy label is unique. You can find the three Blautia clades in the Greengenes2 website if you'd like to explore the taxonomy directly.
All the best,
Daniel