Measured exposure is the time an ad creative was actually rendering on a stream, rather than a count of times it was served; the metering model is built, while the live capture path that reads a running broadcast is still being built.
The unit Publsh is built on: the time an ad creative was actually rendering on a stream, rather than a count of times it was served. An impression counts a load. Measured exposure is about the seconds the ad was on the screen people came to watch.
The metering model that defines and totals rendered time is built. The live capture path that reads every second off a running broadcast is still being built, so measured exposure describes what the system is designed to count and how, not a fully running live meter today. For the full picture, see the measured-exposure pillar.