Currently it is not possible to create a segment to select all impressions for a certain Target Activity. Reason: The Activity Impressions metric cannot be used in the segment builder.
Consequence: you select all visits/pages of a visitor who was in a certain activity, which is not the same as selecting only the impressions of a an activity.
Use case example: on the homepage we have 2 activities, one for prospects and one for customers. When the homepage is loaded, either a visitor is recognized as prospect of customer and sees one version (=activity) of the homepage. We want to introduce a data quality check that checks whether someone is not in both activities. This means we need to be able to create a segment based on the activity impressions.