First is NFC record. But if we both win out we'd have the same. So next up is common opponent. After that it's margin of victory or some shit.
I saw this on Reddit so I can't vouch for its accuracy but it seems right. I'm just copy/pasting:
It would actually be best for ATL to lose both of those games. The only way (barring ties) that SEA, DAL, or DET make the playoffs is to go 10-6. If ATL loses both games, ATL will be 9-7, so the tiebreakers don't matter.
For SEA & DAL, it doesn't really matter which team does it. Both have the conference record tiebreaker over NO & CAR, and since ATL would be 9-7, that's irrelevant, too. However, if DET is also at 10-6, then DET would actually win the tiebreaker over SEA/DAL on common games (NO/CAR would drop out on conference record). So SEA/DAL needs DET to lose a game as well.
For DET, it would be much better for ATL to lose out, because DET has head-to-head losses to all three NFC South teams. If DET is stuck in a 2-way tie with any of them, DET is out. However, if DAL or SEA also finish 10-6, then that allows DET to bypass the head-to-head loss (because head-to-head only matters in a 3-way if one team lost to the others or one team beat all the others, and neither DAL nor SEA played DET, CAR, or NO). So if NO or CAR is the team that loses, out, DET requires the winner of the DAL/SEA game to win week 17, too. But if ATL loses out, DET clinches the berth no matter what DAL/SEA do because DET would have a better record than ATL and would have common games on DAL/SEA.