Electronic transmission is not, by itself, filing of the return. The Polish administration states that the UPO is official proof of receipt; the reference number assigned on sending initially serves to track processing.
The distinction is practical: status 200 enables retrieval of the UPO, while intermediate processing statuses do not yet demonstrate successful filing. A rejection status requires investigation of its cause. Keeping only the reference number therefore leaves filing evidence incomplete.
Internal checks can accordingly examine the match between the file sent and the acknowledgement ultimately obtained. Evidence relating to a different period or return would not resolve the difficulty in the filing under examination. It therefore appears preferable to keep the return, its reference and its processing outcome together; this organisation supports proof of receipt without prejudging the accuracy of the declared tax base.
This vigilance is particularly important during electronic-service maintenance. An announcement of unavailability does not, in itself, extend a tax deadline. The processing outcome and UPO corresponding to the return actually received should be retained, without confusing receipt with substantive validation of the declared data.