Usually the ad needs to be in your viewport for at least a few seconds to count as an impression. If you were just going back quickly, or quickly refreshing the page, it won’t count. If you go back or refresh, see a different ad, wait a bit, then refresh again, I think it’d count.
For skippable ads on YouTube, the advertiser only pays if you watch past the point where you can skip it. If I remember correctly, you have to watch at least 30 seconds of the video (or the full video if it’s less than 30 seconds) for it to count as a view.
Does it also cover reloading with different ads? (such that it would count impressions for different advertisers)
Usually the ad needs to be in your viewport for at least a few seconds to count as an impression. If you were just going back quickly, or quickly refreshing the page, it won’t count. If you go back or refresh, see a different ad, wait a bit, then refresh again, I think it’d count.
For skippable ads on YouTube, the advertiser only pays if you watch past the point where you can skip it. If I remember correctly, you have to watch at least 30 seconds of the video (or the full video if it’s less than 30 seconds) for it to count as a view.