Tài liệu PL/SQL User’s Guide and Reference phần 3

Until you initialize it, a nested table or varray is atomically null (that is, the
collection itself is null, not its elements). To initialize a nested table or varray, you
use a constructor, which is a system-defined function with the same name as the
collection type. This function constructs collections from the elements passed to it.
In the following example, you pass six elements to constructor CourseList(),
which returns a nested table containing those elements