what does a model number means in camera?

Answer 1

As with most store bought items, the model number primarily identifies the period of manufacture and possibly the range of features incorporated within it.

For example model: 'SLR-A' might be older than model 'SLR-B' - and because model 'SLR-B' is more up to date, it may have had features added or removed.

Hope this helps

Answer 2

it tells you which camera is which