Fill rate is the ratio of ad views to video views. Sell through rate is the ratio of ad views to ad opportunities.
Fill rate can be higher than one. For example, if a user watches 1 ad, then 1 video, then 1 ad again, but never finishes the ad, then you have 2 ad views and 1 video view. The fill rate will be 2.
Accordingly, sell through rate is the most accurate and true measure of monetization against available opportunities. It is tracked by our player, it accounts for user experience caps to reduce ad frequency, and it measures all demand (including FROS, etc.).