https://www.rfc-editor.org/rfc/rfc8228.txt

Abstract

   Rules for validating identifier labels and alternate representations
   of those labels (variants) are known as Label Generation Rulesets
   (LGRs); they are used for the implementation of identifier systems
   such as Internationalized Domain Names (IDNs).  This document
   describes ways to design LGRs to support variant labels.  In
   designing LGRs, it is important to ensure that the label generation
   rules are consistent and well behaved in the presence of variants.
   The design decisions can then be expressed using the XML
   representation of LGRs that is defined in RFC 7940.