Difference in source accounting lines and project accounting lines

Summary:

From prior experience, in project costing when dealing with costs from a different subledger (Payables for example) I have only ever been prompted to see the Source Accounting Lines transaction. In my new engagement I am finding that there is both the source and project accounting lines provided. What are the setup(s) behind this functionality?

Example:

Expense coming from payables

Source Accounting Lines: Supplier Invoice xxxxxxx

Dr :100 Item Expense

Cr: 100 Liability

Project Accounting Lines: Supplier Invoice xxxxxxxx

Dr: 100 Burdened Cost

Cr: 100 Burdened Cost Clearing

What is the purpose of two different accounting lines? I have only ever seen the source if coming from payables and not the project line.