The top of the plate where food is placed is called the "well" or the "food- contact surface." The well refers to the bottom part of the plate, which is the flat area designed to hold the food. This surface is critical for proper plating and presentation of the food and must be kept clean for hygiene reasons.
Additionally, the term "food-contact surface" specifically denotes any surface where food directly contacts, which includes the top area of the plate.
The plate also typically features a "lip," which is the flattish raised outer part, sometimes incorrectly called the rim, and the "rim," which is the outer edge, often decorative, but food is placed on the well.