What is compatibility styles?
Compatibility style is a stylesheet that only gets loaded if it's file's name and header's
theme name matches the current active theme on your site, i.e.:

current active theme's folder: AvadaCar-child
current active theme's stylesheet's theme name: Avada Car Child

so the compatible stylesheet should be named: AvadaCar-child.css
and it's theme name in the header should be: Avada Car Child

This style is needed for example, when you are not satisfied
with the plugin's current style and you want to make changes. This is a
good way to keep things organized and your custom style won't get lost
after an update. So basically this is a child-theme for your plugin