This setting allows you to enable different degrees of required account verification. "Disabled" means that there is no form of account verification present on your website. Selecting "Allowed" gives new users the option to prove that they are an existing customer through account verification. If you select "Required", a user will not be able to create an account unless they have already done business with your company.

After the setting has been enabled, two different registration options will be available adjacent to the Username and Password fields on your website's login page:

The text underneath both the Existing Customers and New Customers sections are customizable. To customize the text there, navigate to Site Manager > Settings > Users > New Account Setup (see screenshot below).

You can add a third option below the two default options by enabling the Additional Option setting. This setting can be found by navigating to Site Manager > Settings > Users > New Account Setup. By clicking the Design Manager link in the Enable Additional Option setting panel, you will be directed to the appropriate Admin Design Manager template to customize what is displayed there. 

Should a user indicate they are an existing customer of your company, they will be directed to input some required information that will confirm their identity.  In addition to the standard Name and Email fields, the user is also required to supply an Invoice Number from an invoice no older than 30 days; they are also required to supply the Customer Code and Zip Code attached to that specific invoice.  If all three fields are consistent with current records, the user can continue with the standard account creation process.