In the Campaign group settings you must write the target impressions under Paced target. After setting the Paced target you will need to choose between two functions under Target distribution.
If you do not change it from the default None the shared target impression will not be divided between the campaigns in the group.
The Availability adjusted function means that Cxense Display will optimize how the impressions are divided between the member campaigns according to the other campaigns running on the same Content units.
As a standard Cxense Display will always try to divide the impressions equally but if you are running a lot of campaigns Cxense Display might not be able to do so and in such a case it will divide the impressions between the campaigns in the most beneficial way.
This mean that if e.g. you are creating two campaigns in a Campaign Group and one of them is placed on a Content unit, which is already heavily booked Cxense Display will take this into consideration and divide the impression so that the campaign(s) running on heavily booked Content units will receive fewer impressions than the other campaigns in the Campaign group.
Same for all means that Cxense Display will try to divide the paced target equally among all campaigns in the group. Because the forecast cannot be 100% precise you might experience that in some cases one of the campaigns will deliver slightly more than predicted on the behalf of the other campaigns but these fluctuations should only be minor and percentage wise campaigns will deliver almost identical.