A domain is a set of permissible values that can be given to an attribute. So each attribute in a table has a definite domain. Values to these attributes cannot be assigned outside their domains.
In the example given above if domain of PERSON_ID is a set of integer values from 1 to
1000 than a value outside this range will not be valid. Many other common domains may be age between 1 and 150. The domain can be definite by assigning a type or a format or a range to an attribute. For instance, a domain for a number 501 to 999 can be particular by having a 3-digit number format having a range of values between 501 and 999. Therefore, please note the domains can also be non-contiguous. For instance, the enrolment number of IGNOU has the last digit as the check digit, therefore the nine- digit enrolment numbers are non continuous.