From the Campaign tab, you are able to select how this campaign will be billed: By Campaign or By Project.
This allows you to create a Fixed-Fee invoice based upon the Campaign Budget using the Bill Campaign button. When using Billing Worksheets, this will create a single Master worksheet for the Campaign and individual worksheets for each project. The type created will depend on the project settings.
Billing will be based upon the individual projects. If using Billing Worksheets, the settings on the Client record will determine how the worksheet(s) and invoice are generated.