ProMar 200, 400 primer as ceiling paint

Hi. I'm doing some painting projects for the first time. Ever. I'm starting with repainting a sun room ceiling, but hoping to find a product, and learn technique, to use for ceilings going forward. My local Sherwin Williams has Premium Ceiling paint for $58/gal. But I also see others have recommended either Promar 200 ($67) or Promar 400 ($38), each of which which i only see on the SW web site as a "primer." Is this what people are talking about? Are they using primer as ceiling paints? And what are the differences between 400 and 200?

Thanks!
https://www.sherwin-williams.com/homeowners/products/promar-400-zero-voc-interior-latex-primer