In a typical "focus group" situation, test subjects are shown various advertisements and then they are asked about the commercial or print ad that they've just viewed. Their reactions/thoughts/opinions are recorded. A "recall test" as described in your first underlined example is one where the test subjects are not actually shown an advertisement, but are simply given the name of a product and then asked for their reactions/thoughts. The researchers are trying to see if the group answers this question with "Oh, yeah, that's the one with the commercial with the kids singing..." or "That's the beer with all those sexy girls in the magazine ads...." They are trying to determine which ads the subject has seen in the past have stuck in his memory.
Your second underlined passage explains the reason advertisers want to know the above. When a consumer is wandering the aisles at a grocery store and sees 20 different types of breakfast cereal on the shelves, each manufacturers is relying on the consumer to choose its brand. Whether consumers choose Wheaties because of the athletes on the front of the box, or Fruity Pebbles because they remember seeing the Flintstones featured in the TV commercials, manufacturers want to know what causes the consumer to choose a particular brand. They will then spend their advertising dollars accordingly.
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