A flat is usually advertised by its largest number. That number is almost never the space you will live in, and the gap between it and the floor you can actually use is routinely a quarter of the total. Knowing which of the three you are being quoted is the difference between comparing two homes and comparing two marketing decisions.
The three numbers
WordCarpet areaThe usable floor area inside a flat's walls: rooms, kitchen and bathrooms, without outer walls, shafts, balconies or terraces. is the floor inside your walls, the part a carpet would cover. It excludes the thickness of the walls themselves, and it excludes every shared space in the building.
WordBuilt-up areaCarpet area plus the area under the flat's walls, and often its balconies. It has no legal definition, so builders count it differently. is carpet area plus the walls that enclose your flat, and usually the balcony. It is a construction measurement rather than a living one.
WordSuper built-up areaBuilt-up area plus a share of the building's common areas, such as the lobby, lifts and stairs. Also called super area; the largest of the three. is built-up area plus your share of everything the building holds in common: the lobby, the staircase, the lift shaft, the corridors, sometimes the clubhouse and the generator room. This is the number most price-per-square-foot quotes use, because it is the biggest.