Really, you can do anithing you want/need. This aply if for example, you return 'F' or 'M' for identify the sex of a person. The important thing is do a good design and separate rigth the roles (is best not mix in the same class(except if is a composite one, like the root class) the data&logic&validation&Gui). Anyway, for a draw program the "bussines class" are the class to draw but for accounting are the account, bank, etc... The bussines clases can be simply the logic unit to do the direct job of a system (draw, managment, web...)

For example:

1- Pure Bussines Classes: Account, Person, Bank. Pen, DrawBoard. Security Manager clases
2- Helpers: Encrypt, Rule validators, etc..
3- Persisters: Data connection, datasets, transactions...
4- User Interaction: Edit controls, Grids, Etc...
5- Others: Window managment, Aplication services, etc...

If you do a good design and comply with the task well, don't worry soo much...