[1] IDNA のラベルに関する要件をすべて満たす文字列はIDNA 妥当であるといいます。 >>2
[7] IDNA に対応したアプリケーションや文脈ではこれら3種類だけを認めることになっています。 >>2
[8] RFC 7542 - The Network Access Identifier ( 版) <https://tools.ietf.org/html/rfc7542#section-3.2>
The NAI does not make a distinction between A-labels and U-labels, as
those are terms specific to DNS. It is instead an IDNA-valid label,
as per the first item in Section 2.3.2.1 of [RFC5890]. As noted in
that section, the term "IDNA-valid label" encompasses both "A-label"
and "U-label".