These are separator tabs, with no other function than making the UI cleaner. If you have a custom Blender theme, for example a light theme, then the dark gray colors will not blend in with the blender theme. To fix this, click on the colors, use the color picker to pick your custom Blender theme color then save the file and use it later.

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It's up for you to decide which one fits your usercase best based on the information given. If you buy the Pro version, you will also get the Lite version.

I'm researching in this direction right now, but I can't promise anything at the moment. If I can come up with a very fast, automated, realistic, custom volumetric solution for Cycles, then I'll add it right away happily, but the 3D clouds will stay regardless since they are very customizable and a good alternative.

It's a known limitation at this point, this is because of how the mapping is working at the moment for the world node setup I created. This can be improved by animating the clouds yourself into differnet directions.

For example: move X, -X or move X, X direction. Or move Y, X or Y, -Y direction. They will all move the spot with the quick shape-changing clouds to a different location which may improve the part of the scene/sky your camera is looking at.

Absolutely not. I believe, after looking at various online material, that volumetric clouds are either insanely hard to make look good in Cycles (intensive setup and manual modeling required), or in Eevee they never look good in my opinion. Although 2.93 LTS improved Eevee volumetrics so there is a possibility to create a lot better volumetric clouds now.

This shader is intentionally not using volumetric clouds, instead it is using 3D clouds which are technically 2D clouds having dynamic, correct lighting. They are faster to render in Eevee and Cycles, and are on par with the better looking Cycles volumetric clouds.

Although of course they have their weaknesses too since they aren't really volumetric.