eCPM is an estimated CPM offered so buyers can compare against other channels; Publsh defines one impression as one viewer present for 30 continuous seconds of render. It is a translation for comparison, derived from measured exposure (live capture still being built).
An estimated CPM, offered so buyers can compare against other channels. A persistent banner has no natural impression boundary, so Publsh defines one impression as one viewer present for 30 continuous seconds of render, publishes that convention, and does not change it silently.
It is a translation for comparison, not a truth about the medium, and it derives from measured exposure, whose live capture path is still being built.