If an advertiser adds affinity audiences and topics to the same targeting group, a TrueView ad will show:
- only on targeted affinity audiences
- only when targeted topics and affinity audiences match
- on targeted topics and affinity audiences
- only on targeted topics
By targeting your video ads on YouTube and the Display Network, you can advertise to consumers at moments that matter. With a wide variety of targeting methods available to you, such as demographic groups, interests, placements, and remarketing lists, you can reach specific or niche audiences based on who they are, what they’re interested in, and what content they’re viewing. It’s important to know that with each targeting method you add you narrow, not widen, your targeting.
If we use affinity audience, the ad may be shown to the visitor, having immense interest in our product. In this case the can be shown on the pages irrelevant to our keyword, because we are targeting the audience. The audience who has seen the relevant pages earlier and shown the interest to similar products. That is why they are being added in affinity audience. Now, when we target topics, the ad will be shown on the relevant pages. Because we are targeting topics, that are associated with page.
Read more here: https://support.google.com/partners/answer/2454017?hl=en