App user's email vs User Profile email

In fields in the layout or workflow editor, we always have access to data points from the user profile, and at times (I’m not too sure when) we also have access to another set of values: current date and time, unique identifier, app user’s email, link to current screen, etc.

What’s the difference between App user's email and User Profile email?

No difference, really. Unless you don’t have User Profiles configured, I guess.