I didn’t include em or rem units because they aren’t usable in Wix, I tend to use rem when writing CSS although there’s much debate on if it’s actually any better for accessibility compared to pixels. In any case, they operate under the same principals as pixels but are based on the font currently in use. I suspect Wix doesn’t make them available to keep things a little simpler for the end users.
As for when to use each unit, you’ll likely be using both min and max fixed units and percentage based responsive units all over the place. There isn’t really a fixed guide for doing things and you’ll have to choose your units based on what you are making and how you want it to react to the page resizing. With a solid grasp on all the concepts mentioned above you should be able to intelligently choose what units to use where in order to create what you want to.