A work item is created for each interaction that takes place. Additionally, service items are created as part of the interaction and linked via property references. Hence, Interaction items and service items do not have a cover-covered (parent-child) relationship. 

So this means the Interaction case could get resolved even before the Service case is closed isn't it? (Of course if requirement demands only...!!)

Yes. My phone call interaction case can be resolved first before the fraud complaint service case is resolved.