Idk and idn
What is composed rating?

There are many combined chess variants in Variants. One can call them variant mixes. For example, you can play a combination of Spell Chess and Duck Chess. When you play such a variant, you earn rating points for both these variants, and appear on both leaderboards. Those combined ratings are composed.
There are also variants with custom starting positions, they can be tactical or positional, and have other flavors. If you play positional variants, you earn positional rating points. And so on.

Thanks, BabYagun!
I contacted the support out of curiosity and got the same answer too(even the image was the same). I see you're a chess.com staff too, so maybe that's no surprise lol.
I see this button on chess.com variant site. But what is a composed rating?