What is CDU?

CDU is the abbreviation of "Clasificación Decimal Universal" (Universal Decimal Classification). It is a system that encodes the topic of a certain work with numbers, and that is generally used in the organization of the shelves in most of the universities in the world.

What is the meaning of s.n. in a bibliographic register?

S. n. means "without editor". As a consequence, it is used when the editor of the book is unknown. 

What is an "autoridad"?

"Autoridad" is a name of a person or entity used to designate  this specific person or identity  always in the same way.