Modified Jungle Edge biomes only generate when a Jungle borders a Swamp Hills biome

Modified Jungle Edge (Jungle Edge M) Biomes only Generate if Jungle borders a Swamp Hills (Swampland M) Biomes..

The Light Green is a Modified Jungle Edge and the Light Teal is Swamp Hills.

As you can see, the Modified Jungle Edge Biomes only Generate when Jungle borders Swamp Hills Biome. This makes Modified Jungle Edge the rarest biome in the game.