I was told to add „sans of topic“ to the tags yesterday. Seems like I still have to figure out how this works.

What do you mean subcommunity or topic stream?

Subcommunity: part of a bigger stream
Topic stream: a singular stream dedicated to one topic
(this is my guess)

It's a topic stream then. There ain't any other bigger Undertale stream here...?

A while ago I divided the Imgflip streams in three categories:
1. General streams
These streams are specified for memes, but all types of memes. An example for a general stream is fun.

2. Topic streams
These streams are made for memes of a specific type or topic. Some topic streams are you-had-one-job, cats, dogs, gaming, sports etc.

3. Subcommunities
These streams are the home for communities within the Imgflip community. Subcommunities are often accompanied by a more or less complicated history, iconic users and their behaviors, and some controversy. Some subcommunities are MSMG, Memes_Overload, Vibe_Zone, and Justacheemsdoge.

Oh, I see.
The Understream is potentially is a Subcommunity, as you call it, except we have a complicated history and some dramatic controversy. Everyone here knows each other, and everyday interaction, at least once, is the best way to make yourself a known and popular user.