Accounting documents associated in the creation of a billing document

Hello experts,

When I am creating a billing document, it is getting some FI documents that already have a billing document associated. I am debugging the whole standard and I know that it is related to the creation of records of table AD01DLIEF. However, I do not find the part of the code in which they are assigning the FI documents. I set several breakpoints in the reference of the table AD01DLIEF but so far I did not find anything.

Do you have any idea in which part of the code, user exit or badi I should check?

Thanks a lot