It is not uncommon for customers to need to bill specific tasks together on one invoice but not ALL the tasks on a project together. If we had a bill group field on the tasks (And Task Template to be copied to new tasks on the project) then the user could designate which tasks are billed together
When the 'Create Invoice' process is run then it would create invoices based on the billing group of the tasks. There should also be a radio button on the project card to indicate that the billing method is 'Bill by Task group'
It is not desirable to use separate projects and bill using the Main Project feature. If bill by task group was offered this would give a very broad range of flexibility of the setup of projects, tasks and main projects