Fill is the ad-network model Publsh is built against, where a system drops a creative wherever the algorithm has room and placement is randomized. Publsh is deliberately not a fill network; nothing is randomized.
The ad-network model Publsh is built against. In a fill model, an ad system drops a brand's creative wherever the algorithm has room, and placement is randomized. Neither the brand nor the creator chose the pairing.
Publsh is deliberately not a fill network. Nothing is randomized: a brand picks the creator on purpose, the creator approves the creative, and both sides consent to the placement before it renders.