1. Good question since the average user will be equally puzzled, though I'm not immediately sure how to make that any clearer.
CRE stands for crengine, as in coolreader engine, see
here for the technical mumbo jumbo. That's used by default for filetypes like FB2, EPUB, HTML and a few others. Simply put most formats other than PDF and DjVu.
(If you long-press on a file you can also select MuPDF to render some of those; support for that is currently very proof of concept.)
2. Cover-Image is probably a typo that should be "Cover image". It's a plugin:
https://github.com/koreader/koreader...meta.lua#L4-L5