INV_RECPT_DT vs. INVOICE_RCPT_DT in PS_VOUCHER table

There are two date fields in PS_VOCUER table.

1. INV_RECPT_DT   (this is the only field available in VCHR_HDR_STG table)
2. INVOICE_RCPT_DT (this is the only field which is displayed on the page)

Both of these dates say  Invoice Receipt date.

Both of them are not populated at the same time.  i.e. this query always returns null

select voucher_id, INV_RECPT_DT,invoice_rcpt_dt from ps_voucher

where INV_RECPT_DT is not null and

invoice_rcpt_dt is not null

Can anyone please tell me what is the use of each field?