It describes *both* mirrors on my car and on my wife's car: plane for seeing the periphery of the car (*) and the majority of the traffic in the lanes either side of you, and convex for seeing cars that are very close to you in an adjacent lane and are therefore off the edge of the plane section. I use the convex section to determine that there is a car very close to me when I'm about to overtake or pull in, such that I can't judge its distance away (because it appears further away / smaller than it really is) but I know to look over the relevant shoulder to see its true distance.
(*) eg where the rear wheel is in relation to a kerb when parking, by tilting the mirror down - either manually or else automatically when reverse is selected.