Share of voice is the proportion of the advertising or search real estate for a given keyword that your brand occupies, relative to everyone else competing for it. Amazon does not publish a single share-of-voice figure, so any number you see is derived — either from Amazon's impression share reporting, or from third-party tools that scrape search results and count placements. Treat a specific percentage as the output of one method rather than a fact from Amazon.
The two ways it gets calculated
Impression-based: what proportion of the impressions available for a search term your ads received. This comes from Amazon's own reporting and is the more defensible, because the denominator is Amazon's.
Placement-based: a tool searches the keyword on a schedule and counts how many visible slots belong to your brand. Useful and visual, but the result depends entirely on how often, from where, and on which device the tool searched.
How to use it without misleading yourself
Most useful as a trend on a small set of priority keywords, tracked with one consistent method. Comparing a figure from one tool against another tells you about the tools, not the market.
It is a means, not an end. A rising share of voice on a keyword that does not convert is simply a more expensive way of not selling anything.
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