When creating a new component, if you haven't selected all the entities related to a Pattern/ Mirror feature, the program asks to complete the selection automatically.
The component will include all the entities involved in the feature.
When defining a Cross-component feature, you are not allowed to create it on components including other components. On the other hand, you cannot insert a component used in a Cross-component feature into a new component.
In other terms, both when creating the feature and the component, you are not enabled to create a component that is already a part of a Cross-component feature or create a Cross-component feature involving components of more than one level.