Glossary of Relevant Terms

An "A-label" is the ASCII-Compatible Encoding form of an IDNA-valid string. It must be a complete label: IDNA is defined for labels, not for parts of them and not for complete domain names. This means, by definition, that every A-label will begin with the IDNA ACE prefix, "xn--", followed by a string that is a valid output of the Punycode algorithm [RFC3492] and hence a maximum of 59 ASCII characters in length. The prefix and string together must conform to all requirements for a label that can be stored in the DNS including conformance to the rules for LDH labels. If and only if a string meeting the above requirements can be decoded into a U-label is it an A-label.

As defined in RFC 5890

U-Label

Variant code points across related scripts, e.g. U+0441 CYRILLIC SMALL LETTER ES 'c' and U+0063 LATIN SMALL LETTER C 'c'

An "internationalized domain name" (IDN) is a domain name that contains at least one A-label or U-label, but that otherwise may contain any mixture of NR-LDH labels, A-labels, or U-labels.