A relative distinguished name, or RDN, is a single component within a distinguished name. It comprises one or more name-value pairs, in which the name and the value are separated by an equal sign (for example, for an RDN of uid=ann, the name is uid and the value is ann), and if there are multiple name-value pairs then they should be separated by plus signs (for example, for an RDN of cn=Jon Doe+employeeNumber=12345, the name-value pairs are cn=John Doe and employeeNumber=12345). In practice, RDNs containing multiple name-value pairs (called “multivalued RDNs”) are rare, but they can be useful at times when either there is no unique attribute in the entry or you want to ensure that the entry's DN contains some useful identifying information.
Even though a DN may be composed of multiple RDN components, the leftmost component is typically referred to as the entry's RDN. For example, in a DN of uid=john.doe,ou=People,dc=example,dc=com, the RDN would be uid=john.doe. The attribute values specified in an entry's RDN must be contained in that entry, so the entry uid=john.doe,ou=People,dc=example,dc=com must have a uid value of john.doe.