This term is being used to describe a unique identifier for a domain of codes. I.e - "10" could be "Glomerulonephritis" in the "RR4" coding standard or "CAPD Connect" in the "RR7" coding standard or "Hyperacute Rejection" in the RR10 coding standard. A code within a coding standard should have a definition which may or may not have an exact match in other coding standards.
Some of the UKRR coding standards can be found at Code Lists
Others are internaional standards such as SNOMED and LOINC.
Further codes have been defined for internal UKRDC purposes.- See Code Sets