Wikipedia:
"production-derived vehicles limited in terms of power, weight, allowed technology and overall cost. Group A was aimed at ensuring a large number of privately-owned entries in races."
"To qualify for homologation, a minimum of 2500 cars of the competing model have to be built in one year, out of 25,000 for the entire range of the model (eg: 2500 Subaru Impreza WRX, out of 25,000 Subaru Impreza. Up to 1991, the requirement was a minimum of 5000 cars in one year, without regards to the entire range, but the FIA allowed "Evolution" models to be homologated with a minimum of 500 cars (eg: BMW M3 Sport Evo, Mercedes-Benz W201 Evo). Rules also required some of the interior panels to be retained, e.g. interior door panels and dashboard."
"Group A consisted of three divisions, Division 3 - for cars over 2500cc, Division 2 - for car engine size that are between 1600-2500cc , Division 1 for cars that are less than 1600cc. These cars competed in standard bodykits, with the production-derived nature required manufactures to release faster vehicles for the roads in order to be competitive on the track. Tyre width were dependent on the car's engine size."
"Rallying
Mitsubishi Lancer Evolution VI, Tommi Mäkinen Edition.
Under Group A in the World Rally Championship, the cars used were modified road cars, often based on turbocharged, four wheel drive versions of small cars such as the Lancia Delta Integrale, Toyota Celica GT-Four, Subaru Impreza WRX and the Mitsubishi Lancer Evolution. In order to be homologated, manufacturers must produced 5,000 units worldwide, and then in 1994, the numbers had been reduced to 2,500. The cars are further modified for greater power and torque, and fitted with suspension and tires specifically suited to the conditions of the specific rally, which may take place entirely on bitumen roads, different consistencies of gravel and dirt roads, and even snow-covered roads on some rallies held in northern Europe. Group A is still used as the basis for most rally competitions around the world, but the most competitive cars are limited-production prototypes, known as World Rally Cars and Super 1600. The last WRC car to use the old Group A homologation requirement was the Mitsubishi Lancer Evolution VI."