The top of the plate where food is placed is called the "food-contact surface" or simply the "surface" or "face" of the plate. It is the area that directly comes into contact with food and must be kept clean and sanitized to ensure food safety.
In more detailed terms, according to dishware design, this part is sometimes referred to as the "well" of the plate, which is the bottom part where food is placed, distinct from the "lip" or edge of the plate.
