Notation for (simplifying) "order 100" which (simplifying) means they have anywhere between 100 and 999 concurrent viewers at a time.

It is a good representation for Twitch since that is generally the tier where individual streamers, or even streaming groups, can make a living off of twitch. It is a good indication that they'll have a good number of subs per stream, are big enough to meaningfully partner for ad revenue, and will have good discoverability in the various directories. I forget what the current data is, but last I saw even someone who averages 200-300 concurrents is already the top 1% of twitch streamers and revenue since the vast majority are lucky if they can hit double digits. And the O(100) streamers will often spike into the thousand range if they get lucky on the homepage.

I've also seen speculation that that is the scale that Twitch/Amazon actually want streams to be at since they tend to "pay for themselves". As opposed to someone like Pokimane who tends to range from 10k-30k (depending on the game/how long she has been going) where even getting a hundred subs donated doesn't cover the bandwidth costs to serve the 1080p stream to everyone. And then you also have the likelihood that those streamers are doing sponsored streams where they get a massive sack of cash that Twitch never even get to stick their fingers in.