Ron, as Waynew indicated there are really two things here. To the purists, numbers matching means the car still has the various components with which it was originally built......engine, transmission, rearend, etc. They would also consider it should have the original carb, intake, exhausts manifolds, accessories, paint and trim matching the cowl tag, etc.
Most cars that are called 'numbers matching' are not and should probably be referred to as 'date correct.' This means the various components are not necessarily all the originals, but are correct parts dated within acceptable time frames for the build date of the car. Chevy By The Numbers indicates that 1-3 months prior to the build date of the car is acceptable, although this was frequently extended by labour disputes, strikes, raw material shortages, etc.
Hope this helps.
Bob