IDNA does not apply to domain names in the NAME and RDATA fields of DNS resource records whose CLASS is not IN. This exclusion applies to every non-IN class, present and future, except where future standards override this exclusion by explicitly inviting the use of IDNA.
IDNA applies only to domain names in the NAME and RDATA fields of DNS resource records whose CLASS is IN. See the DNS specification [RFC1035] for precise definitions of these terms.
The application of IDNA to DNS resource records depends entirely on the CLASS of the record, and not on the TYPE except as noted below. This will remain true, even as new TYPEs are defined, unless a new TYPE defines TYPE-specific rules. Special naming conventions for SRV records (and "underscore labels" more generally) are incompatible with IDNA coding as discussed in the Definitions document [RFC5890], especially Section 2.3.2.3. Of course, underscore labels may be part of a domain that uses IDN labels at higher levels in the tree.