What does REVERSAL_FLAG mean in the table AP_INVOICE_DISTS_ARCH ?

Dear Masters,

What does REVERSAL_FLAG mean in the table  AP_INVOICE_DISTS_ARCH ?

It has 4 values:

U

N

Y

Null

what are all means in the table AP_INVOICE_DISTS_ARCH ?

We are going to run the following GDF's to populate Invoice Lines prior to Jan-2009, so what would be the value of REVERSAL_FLAG in the table  AP_INVOICE_DISTS_ARCH post execution.

1. 13692150:R12.AP.B    - re-upgrade invoices  ( which will create the lines for invoices)

2. 14040540:R12.AP.B    - For updating the tax amount at header level of invoice.

Thanks,

Romen