Subprocess
The subprocess in combination with the item type represents the business significance of a billable item.
Each subprocess is assigned to one or more billable item classes. The subprocess is an independent processing branch within the billing process with regard to selection and the grouping of billable items and their processing for a billing document. The system assigns the billable items to one of the subprocesses as soon as they are created. You cannot process these billable items together with the billable items of other subprocesses in a common billing document.
The following control options exist:
Selection of the billing worklist
For each subprocess you can define which billable item classes the system is to select and process. You can further refine the selection by specifying per class the type and source transaction type of the billable item.
Grouping of the billable items in billing units
You can assign to each subprocess an individual grouping variant for automatic summarization of billable items for which the system creates a common invoice.
Definition of the billing type
You create new subprocesses in Customizing for Contract Accounts Receivable and Payable under 
Convergent Invoicing
Basic Functions
Billable Items
Billable Item Management
Define Subprocesses of Billing
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Example
You have two distinct groups of billable items (for example from two different industries).
The system should do the following with these items:
Bill them at different intervals
Select them according to different considerations
Group them into billing units and process them in billing documents
You achieve this by assigning the groups their own subprocesses.
You can then process the two subprocesses together or in separate billing processes.
If business-related dependencies exist between the subprocesses, such as with music downloads and royalties, it may be useful to assign both subprocesses to the same billing process. A business dependency in the music download example may occur if, for example, there is a requirement that royalties only be billed once the corresponding music downloads (payables) have been billed.