The 'Instances' metric shows the number of times a dimension was explicitly defined in an image request. Some dimensions, such as eVars , persist dimension items past the hit they are set on. This metric is useful when you want to see the number of times a dimension item was set without the hits where that value persisted.
How this metric is calculated
Out of all hits in a report suite, only include hits that explicitly set a dimension item in the image request. Some dimensions, such as eVars , persist beyond the hit they are set on. Metrics like Page views and Occurrences count both initial and persisted values. This metric does not count persisted values.