Billing types are pulled from the Dynamics GP - Project Series module are used as follows:
Used with Time and materials, fixed price, cost plus Project types when you want to bill customers for project costs based on a specific profit type.
Used with Time and materials Project Types when you won’t charge customers for project costs. For record purposes, you can send a billing to customers to indicate that work has been performed, but the billing amount will always be ”0”.
Used with Time and materials Project Types when you won’t bill customers for project costs. An N/B transaction is not included in the billing process or in the invoice.
When your billing type is N/C or N/B, your total profit, billings and revenues will be 0.
For Additional Information regarding Project Series and Billing Types refer to the GP - Project Series Help Guide.