I’ve barely been near PhotoLab in months for a variety of reasons and so hadn’t seen these, but had I seen them I’d not have had any problem deciphering their use as, with the possible exception of “Rotate to fit” and “Megapixels” they are common options in many graphics programs.
In fact, the “fit” terminology is used widely, even on setting desktop wallpaper images on macOS.
As for why one might want megapixels — well this was actually a request I made to the developer of another graphics program. If I am in the habit of taking 3:2 ratio pictures in portrait or landscape, then exporting them with “3000 pixels longest side” is a fine choice resulting in either 3000x2000 or 2000x3000 pixel images.
However a small number of my photos end up cropped square and neither 3000x3000 nor 2000x2000 are suitable as they subjectively appear “too large” or “too small” respectively. I’d rather they be somewhere in between. As 3000x2000 is 6 megapixels, it seems logical that square images would be exported at around 2450x2450 resulting in roughly the same number of pixels in the photograph.
In actuality, I tend to export at longest side = 3200 so had a separate preset at 2600 for square images. As I tend to export tens or hundreds of images at a time, manually deselecting then later reselecting square images to use a different export preset was a chore.