Sehto is a small volcanic island a way's removed from the shores of Johto and Kanto. It overlaps with the real world (even if there are no statements made where therein it resides) in that references to Africa exist; it overlaps with the pokémon world in that it's about pokémon.
Sehto is freeform, doing away with much of what the pokémon games themselves depend on - levels most notably, but also pokémon stats which are freeformed depending on whatever feels realistic in a given fight. There is no four-move limit, either. Legendaries are nearly unbeatable by definition. Finally, there are two new types to Sehto: Blood and Light, former which is an offensively geared type roughly designed to be your worst nightmare, and latter which is a defensively geared type, notable in the offence mainly due to its effectiveness against Dark.