Each Compartments operation can have one Parent object tag and multiple children object tags. If there are several potential parent objects in your pipeline, either multiple Compartments operations must be used (one for each parent type), or the parents must be given a common tag with the Combine Segment Outputs operation. The tag to be used for the parents can be selected from previous pipeline tags. 

Parents can have multiple children tags. When added to the pipeline the operation starts with one parent and one child tag. To add another child object tag we can just click "+ Add input" and select from the available object tags. Note that the children cannot be either the parents or a subset thereof.