For any given tuple, the actual values of its attributes must be taken from the attributes' domains. A domain is essentially a data type that defines the set of all permissible values. Notice that we said an attribute had to contain one of those values. It couldn't contain more than one. In addition to being constrained to a domain, attribute values have to be atomic.
Relations have other properties as well.
Most important, they have the mathematical property of closure. That is, any operation performed on a relation yields another relation. This lets you perform mathematical operations on relations with predictable results. It also allows operations to be abstracted into variable expressions and nested. In his original paper, Dr. Codd defined a collection of eight operators called relational algebra.