Cinderellas: Definition/Groups

'Cinderella' is a term that I find amusing and confusing. It seems there is no clear line between what is and what is not a 'Cinderella'. Some experts include 'Back of the Book' in Cinderellas other do not.

I see the 'Cinderella' stamps falling into 4 basic categories:

1) Communication: Any form of communitcation that required the
use of stamps for service. (Postal, telegraph, telephone)

2) Revenue: Any form of income to a government, person, business or
charity that required the use of stamps to pay taxes, fees,
fines, donations and non-communication services. (Xmas seals,
duck stamps)

3) Advertising: Any stamp so designed to promote a person,
a product, a place, an event, or an idea. (campaign, tourist)

4) Artstamp: Any stamp created, in the this format, as a form of
personal expression.

Now there are always crossovers, but I believe these to be the minimum
basic breakdown of 'Cinderella.'

What do you think?