Meaning of "numbrs matching" opinions please

[SS4Real]

Dale McIntosh's opinion as quoted on the Team Chevelle website and the best definition I have seen:

1. Engine block should be original to the car and where applicable, the engine ID and partial VIN of the car should be present and not restamped. All appropriate external equipment should be on the engine. For example, if it came with smog equipment (A.I.R.) then it should have the smog equipment especially if the engine is coded as such.
2. Heads don't have be original to the car but should be of same type and casting dates should be appropriate. Naturally it's easy if the heads are original.
3. Intake should be correct casting number and casting date.
4. Carb, distributor, exhaust manifolds should be correct casting/part numbers and casting dates.
5. Obvious items that are replaced due to wear and tear such as water pump, alternator, p/s pump, a/c compressor, plugs/wires/oil filter etc should be correct type. Rebuilt alternators are available with correct numbers and dates however.
5. Transmission and rear should be original to the car and have the correct ID and casting numbers.
6. I may differ with others on this one but the paint color(s) and interior should match what is on the Fisher Body plate as well. If the car originally was Sierra Fawn for example with tan bench seat interior and has been repainted resale red with black buckets, the car no longer matches the numbers on the body plate.