What constitutes an 'Open Class' model and what constitutes an 'Individual' entry?

For instance, with the newer ARTF models I can buy them painted with scale gear etc. I believe that this would still be eligible for 'Individual' status. If I buy the same model and have it painted commercially would it be 'Open' class? If I buy aftermarket 'scale' undercarriage it would still be 'Individual' but if they are made specifically for my model then I am not sure where I would stand.

What are the accepted rules regarding the classes? The IJMC rules are not too clear. Does it matter? Are there any differences between the classes?