The Visibility Score feature is a core metric which assists in highlighting data issues and/or optimisation opportunities. Opportunities that are fundamental to any digital marketeer who strives to improve their data performance.
In a Google Product Feed, within the Performance UI you'll now have access to a Visibility Score window. 
WHAT IT'S USED FOR
The Visibility Score shows how your products have performed in two ways:
- the average score for the Partners set timeframe (in this example 15 days) vs the previous timeframe before that
- and the latest (last 24 hour period) vs the set timeframe
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In the example below, the visibility score is indicative of a downward trend in the number of products that are receiving impressions every day during the last 90 days.
The above graph shows a downward trend in the Impression count
The above graph shows a downward trend in the products that are visible
The above graph shows a downward trend in order count, Revenue and ROAS.