CDDL contains five types of field description  statements.   You  can
  use  an  elementary,  STRUCTURE, COPY, VARIANTS, or VARIANTS_OF field
  description statement, but you can use  only  one  field  description
  statement to define a record.

  STRUCTURE,  COPY,  VARIANT,   and   VARIANTS_OF   field   description
  statements   are   subdivided   and   defined  by  subordinate  field
  description statements.