File authors.xml: author vs. contributor

File data/authors.xml contains a list of people having contributed to Gramps. This list is displayed in the About dialog with button Credits.

People are qualified with an attribute title=“x” where x can be author or contributor. What is the difference?

E.g. Paul Culley @prculley and Nick Hall @Nick-Hall are qualified “contributors” while perfectly unknown (nowadays) others are “authors”. I understand perfectly that Don Allingham is the initial author but who are the others?

Is there any rationale under which qualification to add a new contributor/author?