R12 : XLA_AE_LINES .source_table and XLA_AE_LINES.source_id columns are NULL ??

Hi there,

We need to understand under what scenarios are the columns  XLA_AE_LINES .source_table and XLA_AE_LINES.source_id  populated for a PURCHASE INVOICE.

We created a purchase incoice and created  accounting for it.

We can see a records created in XLA_AE_LINES for it.We were expecting to see the columns 

 XLA_AE_LINES .source_table  to be populated with  ('AP_INVOICES'   or   'AP_INVOICE_DISTRIBUTIONS')

and 

XLA_AE_LINES.source_id  to be populated with INVOICE_ID.

Other column have values like below :

1.Accounting_Class_Code = LIABILITY

2.GL_TRANSFER_MODE_CODE = S (Should mean Summary)