When scholars talk of Paul, who are they talking about?

Some scholars say that "Paul" did not write Ephesians , whereas they are confident that he did write Romans. Who are they talking about when they say that? Some quick possibilities could be - The author of Romans - The Paul who Luke speaks of - The early Christian missionary to the gentiles - The Paul who Peter speaks of Now, obviously, if one is defining Paul as the author of Romans, then the deduction that Romans is a genuine letter of Paul becomes trivial. So, my question is "what are the trivial facts about Paul?" or "if scholars had to replace the word 'Paul' with a description then what would that description be?"