The serving log is the second-by-second record a flight closes with, what creative ran, in which placement, when, and for how long, read on the serving side and designed to be the source of truth for billing so unrendered time is not billed.
The second-by-second record a flight is designed to close with: what creative ran, in which placement, at what time, and for how long. It is meant to be read on the serving side rather than claimed by the creator or inferred from a screenshot.
The serving log is designed to be the source of truth for billing. When a stream drops or a card comes off screen, the log records that the ad was not rendering, so that time is not billed. It is how a brand can read a flight line by line instead of taking a total on faith.