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.
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.