Another possibility is `.zz`, which technically can be a ccTLD but it's a user-assigned ISO 3166-1 alpha-2 code, and its last position makes extremely impossible for it to be repurposed as a valid code even in that setting. In comparison, some user-assigned codes like `XZ` are often used for temporary country codes so `.xz` would be less appropriate.<p>It seems that ICANN did consider this choice among others, but reject for the lack of meaningfulness:<p>> The qualitative assessment on the latter properties was performed in the six United Nations languages (Arabic, Chinese, English, French, Russian and Spanish). [...] Many candidate strings were deemed unsuitable due to their lack of meaningfulness. [...] In this evaluation, only two candidates emerged as broadly meeting the assessment criteria across the assessed languages. These were “INTERNAL” and “PRIVATE”. Some weaknesses were identified for both of these candidates. [...]<p>I wonder if this means that they only scored the highest among others and <i>all</i> candidate strings were indeed unsuitable, but that they had to pick one anyway. I'm not even sure that laypersons can relate `.internal` with the stuff for "internal" uses.