What Thom is saying, is basically this:
Frenching is the act of recessing or moulding a headlight, taillight, antenna or number plate into a car body to give a smoother look to the vehicle. The name originates from the end result looking like a French cuff of a shirt sleeve, which has a ridge at the end. Also known as tunnelling, it is a common modification used on leadsleds and customs since the 1930s
It's used more to describe headlights/tail lights than a number plate recess. Though in both cases, it would still be the correct way to address the modification.
