nhd file for a world and then your sims travel back to that world, a new instance of that world will spawn, with the world itself in its original state.
That file contains the data for that world as it's been played: the sims' ages and whether any have died or immigrated in, whether and how much the tombs have been explored, any changes you've made in Edit Town, etc. nhd file for each world that has been played within that save.
Inside the folder for a given save is an.
