The basic idea behind open account, is that a buyer can checkout without paying. This would be used by a merchant who is trying to have a single website for both b2b and b2c. Some customers have additional requirements. Here is an example of a custom business rule that could be written: iif(thisCustomer.zip='90210',.t.,.f.)
This tag would be inserted in the 'advset' field of store settings, and would tell the system to only allow customers with the ZIP Code of 90210 to be offered Open Account as a payment option.