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The search query volume refers to the number of impressions that an account was eligible to receive during a given time period. It is further split into search query volume and display query volume. As the name suggests, search query volume is the eligible impressions on the search network and display query volume is impressions on the display network. 

You can use search query volume to investigate cases where there is a drop in traffic or conversions. It can help figure out if traffic or conversions fell due to a lower number of searches on a particular topic or if there is something else that needs to be fixed in the account. To investigate further, it is a good idea to look at the 'root cause analysis' because that tells you exactly which element in the account saw the highest change in performance. 

The Search Query Volume in the PPC Investigator is calculated using the Impr. Share metric.

For example, let’s say we have 100 impressions and a search impression share of 50%. To calculate the search query volume we’ll use the equation of 50% of X = 100, where X is the search query volume.

A change in query volume can be caused due to external factors like seasonality or demand for the product/service. For example, if you're selling Halloween costumes you'll see a drop in search query volume in November vs. October because fewer people are looking for Halloween costumes after the holiday is over. 

A drop in search query volume can also happen if some high traffic keywords in the account were paused.

In the Root Cause Analysis, the positive/negative movers are the account items (Campaigns/Ad Groups/Product partition/Keywords, performance, etc.) that had the highest increase/decrease in the metric that is being investigated. The system tries to go show the most granular combinations that have had a significant change in performance. This is why you may see a keyword+ad group combination listed or even a whole network.