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

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.