The rationale is that tether is a printed supply stablecoin with a fixed value; there is no need to include its value in a combined market cap. It's debatable if that's true, but that doesn't explain what this indicator is.
The second part of this indicator is highlighting when the aforementioned adjusted market cap is above or below certain values. They are round numbers; a psychological factor. A psyop, if you will.
Options to configure are the lower and upper bound and colors. The alerts must be manually turned on, as usual.
This update is mainly to make a note on the utility of this: while it is accurate now, eventually the market cap will explode in a direction, likely to the upside. Take heed of the trend.
Sorry if I haven't replied to your message yet, I'm a bit backlogged :)
In true TradingView spirit, the author of this script has published it open-source, so traders can understand and verify it. Cheers to the author! You may use it for free, but reuse of this code in a publication is governed by House Rules. You can favorite it to use it on a chart.