When you say "DPO"...what do you mean?

TtcMommyof1 • Wife to a wonderful man ❤️. Mommy to an amazing 6 year old girl 💕. Due 11/8 with a baby boy after 37m ttc💙
For example, my fertile week ended the 27th. My most fertile day was the 25th. Am I 4 DPO (from the most fertile day), or 2 DPO (from my last fertile day of the week?