You can attach a competency, learning objective, or activity to multiple parents. This is ideal for situations where the same learning objective is relevant to two or more competencies.
For example, one competency called “Clarinet” and another called “Saxophone” are both reed instruments; in learning to play either, a person needs to learn how to store, maintain, and handle a reed. But once you’ve learned to do this for one instrument, you know how to do it for the other. It makes sense to create a single learning objective, “Store, maintain, and handle reeds,” and use it in both competencies. The person who completes this learning objective in the context of either competency completes it for both.
For activities, you can use the same activity to evaluate two or more different learning objectives. In some situations it might be difficult to devise an activity that clearly differentiates between different learning objectives. For example, you might have a “Conversational German” competency that included the learning objectives “Understand German” and “Speak German,” among others. A natural activity might be to have people actually engage in a conversation in German, yet in this case the activity will test both their ability to understand and speak the language. Here it would make sense to attach the same activity to both learning objectives.