В домене значения атрибутов должны быть атомарны.
Из книги: Learning PostgreSQL
The formal relational model puts a constraint on the domain: the value should be atomic. Atomic means that each value in the domain is indivisible. For instance, the name attribute domain is not atomic, because it can be divided into first name and last name. Some examples of domains are as follows: • Phone number: Numeric text with a certain length. • Country code: Defined by ISO 3166 as a list of two letter codes (ISO alpha-2) and three letter codes (ISO alpha-3). The country codes for Germany are DE and DEU for alpha-2 and alpha-3 respectively.