Yellow vertical line in YouTube video timeline

Question

What does the yellow vertical line signify in the YouTube video timeline?

Is it an ad start point, or is it its you-can-play-now point as many believe?

It's not clickable nor does hovering over it do anything special. What it is there for?

References:

• What is the little, yellow, vertical line that is under the progress bar?
• What about the yellow line in the timeline in videos?

Answer 1

It marks the point when an ad pops up in the player.

Here is the marker visible in a video:

And here is the popup advert when the player reaches the marker: