Hi,
I assume you are talking about status groups but not the strategy groups
The relevance of Billing document incompletion indicator is, at the sales order level, if any information which is relevant for Billing procesing is missing, then system will message for completion of that, this can be either your PAYMENT TERMS, Or BILLING DATE
If you see the status of the order (VBUP), in the Item level, you can see whether the Billing status for that particular item is completed or not can be driven based on this status group assigned
hope it is clear
regards,
santosh