An A-Label is the ASCII-compatible encoding (ACE) form of an Internationalized Domain Name. Each A-Label consists of two parts, beginning with the Internationalized Domain Name for Applications (IDNA) ACE prefix ("xn--"), which is followed by a string that is valid output from the Punycode algorithm. The maximum string length of the Punycode algorithm is 59 ASCII characters in length. The prefix and the string must conform to all the requirements of for the DNS protocol, including the naming rules for LDH labels.
An A-Label only exists in the context of IDNs, which also require a U-Label. Therefore, a string must be able to be converted into a U-Label in order to be an A-Label.