IDN.IDN is an abbreviation for an Internationalized domain name whose Top level domain (TLD) is also internationalized, which could be transliterated .com, .net, .org, etc. or be groupings relevant to the language at hand.
Concerns and considerations.
As this is an issue still being "decided upon," (for many years, apparently) there are many factors to be considered. Here are a few just thrown out to be edited around and made to look pretty.