The Field Zone (originally named the Field Card Zone) is the location on the playing field where Field Spell Cards are activated or Set. These cards cannot be activated or Set in a Spell & Trap Zone. This Zone is located in the top left corner of a given play mat.
If a player already controls a Field Spell Card, they can send their current Field Spell Card to the Graveyard by game mechanics in order to Set or activate another Field Spell Card from their hand. Previously, the old Field Spell Card would be destroyed, but this is no longer the case.