Z Car Club Association guidelines count how many modifications your vehicle has to classify it.  A modification is anything that is both visible (without crawling under the car) and non-stock.  Each of the following examples would count as one mod (e.g. vertical doors + smoked headlights + chrome rims + aftermarket radio = 4 mods total):  

Exterior:  Non-stock wheels, mud flaps, air dam, rear spoiler, bumper, side skirts, mirrors, fender flares, hood, vertical doors, sun roof, convertible conversion, muffler, brakes, headlights, tail lights, marker lights, auxiliary lights, badges, stripes, lowered ride height, paint.

Interior:  Non-stock stereo, speakers, amplifier, boost controller, carpets, upholstery, door panels, seats, seat belts, dash, console, gauges, roll bar.

Engine:  Non-stock turbochargers, intake, intercooler, header, valve cover, strut tower / chassis stiffening bars, engine covers, fastener kits, reservoirs / catch cans, mirrors, strut mounts, paint.