"Requirements for Internet Hosts Application and Support" which clarifies or changes the specification of protocols given in earlier RFCs.
RFC 1123 defines the terms "MUST", "SHOULD", "MAY", "unconditionally compliant", "conditionally compliant". Capitals are used to emphasise that the official definition of the word is being used.
MUST or REQUIRED means an absolute requirement for conformance.
SHOULD or RECOMMENDED means the item can be ignored under certain circumstances, although the full implications should be understood.
MAY or OPTIONAL means the implementor can choose, usually depending on whether it is needed or not.
Something "unconditionally compliant" meets all the MUST and SHOULD requirements, "conditionally compliant" meets all the MUST requirements and "not compliant" - does not meet some MUST requirement.
For example, RFC 1123 amends RFC952 to say software MUST handle either a letter or a digit as the first character of a hostname