Sometimes you want to synchronize entities that are connected to other entities. These relationships can be references between entities, but they can also involve direct parent-child relationship between game objects, or more nuanced use cases.
Here's a guide for what technique to use, depending on the situation:
If you have an entity that needs to keep a nullable reference to another entity, use a normal Entity reference. This includes any existing MonoBehaviour that has GameObject or Transform fields that you want to synchronize over the network.
If knowing about an entity implies that you should also know about some other (connected) entity, you might need to use the ConnectEntity component.
If the entities are placed in a hierarchy, use the techniques for parent-child relationships.