What it is
Right now, two different numbers can end up in Pigment for the same month - one straight from AWS, one via D365 - and nothing checks that they agree. A proof loop adds a checking step in the middle: before either number reaches Pigment, it is compared against the other side, using a shared reference tag on each batch so we know we are comparing like with like.
Why it matters
It is the same idea as reconciling a bank statement. You do not just trust the number, you check it against the other record and only worry if they do not match. Today a mismatch is only found if somebody happens to notice, sometimes years later, as already happened with carrier settlements. With a proof loop, a mismatch is caught automatically and queued for someone to look at, before it quietly becomes next month's opening balance.
What it would take
A place for both sides to land their numbers side by side - most naturally the lake layer already being discussed - a shared reference tag travelling with each batch so the two sides can be matched, and a simple check that flags anything outside an agreed tolerance. None of it depends on picking the aggregation option first.