What is credential???

A credential is a piece of any document that details a qualification, competence, or authority issued to an individual by a third party with a relevant or de facto authority or assumed competence to do so.

a quality, skill, or experience that makes a person suited to do a job : a document which shows that a person is qualified to do a particular job

Explanation:

a biography is a genre that mainly accounts somebody's life and that is written by somebody else. this genre supports sonia nazario's desire to share enrique's life because she can fully inform readers of his entire emotional journey.

in a biography, an author can share the readers almost everything about a person's life, or just one key event or personal aspect the author is interested in telling, like in this specific case in which sonia nazario is centered in enrique's emotional journey.