The rule for determining the domain boundary may be as simple as a data type with an enumerated list of values.
[1] For example, a database table that has information about people, with one record per person, might have a "marital status" column.
This column might be declared as a string data type, and allowed to have one of two known code values: "M" for married, "S" for single, and NULL for records where marital status is unknown or not applicable.
Following the previous example, a Marital Status reference table would have exactly two records, one per allowed value—excluding NULL.
Simple domain boundary rules, if database-enforced, may be implemented through a check constraint or, in more complex cases, in a database trigger.