You are best to should have a Charge Type specific to your Internal process. The Billing Type can be as per a 'normal' job. It can be Output, Input, quoted etc.
Note: The Billing Type of "Internal" is not used.
The one distinguishing factor in the Charge Type that relates totally to Internal Invoicing is:
Internal Sales Detail.
Select Internal Sales Detail on an input based billing type, if you require the cost transactions on the 'Charge To' Job, to be at the same level of detail as the originating cost transactions on the Selling Job. This will provide for the Activity Codes of the original cost transactions, to become the Activity Code on the 'Charge To' cost transaction.
The source cost transactions used to derive the Internal Sales lines, are posted to the "Charge To" job as a single value per Source Activity Code
Note: These Activity Codes do not have to be valid Activities in the Activity Group of the 'Charge To' Job. There is no validation when it creates the transaction in the 'Charge To' Job. This will also apply to Job Work Centres. There will be no validation.