The Hiroshima Accord was a piece of legislation following the Atomic Wars. Precise details are unclear: in the "Dredd's World" map in Judge Dredd Mega-Special #1, which introduced the Accord, it was said that non-Accord megacities "choose not to acknowledge" anything beyond their borders.
Megacities signed up to the Hiroshima Accord included Mega-City One, Mega-City Two, Texas City, Brit-Cit, the Sov Block, Hondo, Simba City, and New Jerusalem. (The latter two are unnamed on the "Dredd's World" map but named in prog 701)
The "Dredd's World" feature mentions the Polar Judges, so Antarctic City may be part of it.