What are the possible combinations between product price and tax?

In the 'Payment Options' tab of the product wizard (product in edit mode), you have an option to check for tax in the "Product price" section.

1. If you leave it unchecked, the product will display the price without any tax.
2. If your tax is setup to zero (in Billing menu > Settings > tab "Currency & Tax"), the product will display the price with zero tax (equivalent to no tax).
3. If you tax is different than zero, the product will display the price plus the tax to visitors or logged clients who do not have a different specified tax.
4. If a logged client has a different tax than your tax (in Contacts menu > Contact Name > link 'Edit Info' in the top right corner > scroll down to the 'Billing settings' section of the client profile form > check the option "I want to use a different VAT" with a different value specified), the product will display the price plus the client's specific tax.