IMH - once "coin of the realm" is exchanged for services then one has moved into the arena of the "pro" - perhaps if we think about the so-called "non-pro" as a "casual pro" it will be clearer. For example:
Full-Time Pro - one who is engaged in providing services on a "generally" full time basis and "generally" relies on it as a primary source of income.
Part-Time Pro - one who uses the business to supplement a more "regular" and mainstream source of income and/or to generate sufficient fiscal resources to make a move like returning to school or to buy a business.
Casual Pro - one who uses the business to generate enough income to take care of a debt, expense or to pay off a loan then generally exits the business.
Over my time in the "hobby" if have encountered all three and each one has her own special reasons for doing what she does.
