One table or two?

I'm trying to decide if I need one or two tables for this issue.

I'm building a Billing/Inventory Tracking solution. A user creates Orders which have children named Bill Items. Those Orders also have children Contracts, which have children Transactions.

Bill Items look up Bill Codes and auto enter description, price, etc.

Transactions look up Inventory Items and auto enter description and current stock.

I'm trying to decide if Bill Codes and Inventory Items should be one table or two. For about 90% of the records, Bill Codes and Inventory Items are the same. But for about 10% of the records, Bill Codes are assemblies of Inventory Items (I have a separate table of Assemblies which would be a join between the Bill Code and Inventory TOs).

I can put Bill Codes and Inventory Items in separate tables. Or I can have Bill Codes and Inventory Items in the same table, and use a field that will have a value of Bill Code, Inventory, or Bill Code ¶ Inventory.

Any thoughts?