In several places the notions of a plain monomorphism or a plain epimorphism are used, but never defined. See eg. here on page 60, or search google with query
plain monomorphisms tholen
Do they mean a general mono, as opposed to extremal or strong or regular?
As you guessed, "plain" just means there are no additional assumptions, as opposed to things like extremal. So "plain monomorphism" means the same thing as "monomorphism".