On-screen seconds is the total seconds a creative actually rendered, summed from the overlay's heartbeat while the channel is confirmed live; it is the raw render time under viewer-seconds, with the live capture path still being built.
The total seconds a creative actually rendered on a stream, designed to be summed from the overlay's heartbeat and counted only while the channel is confirmed live. It is the raw render time underneath viewer-seconds. The metering model that defines it is built; the live capture path that would record it off a running broadcast is still being built.