Generally speaking, if a set contains all the elements involved in the problem we are studying, it is called a complete set, usually denoted as u.
Collection introduction:
Set, for short, is a basic concept in mathematics and the main research object of set theory.
The basic theory of set theory was founded in19th century. The simplest statement about set is the definition in naive set theory (the most primitive set theory), that is, set is "a certain pile of things" and the "things" in set are called elements. A modern set is usually defined as a whole consisting of one or more definite elements.