Please provide more clarification. Observation.focus can reference any resource. If the observation is about a relatedPerson, clarity is needed for the following: 

1. If both Observation.subject is provided and Observation.focus is a relatedPerson what does this mean. Clarification and/or guidance is needed.
2. Should focus be a modifier element because it changes the understanding of what the observation is about?

Clarity is needed. 

Use case: RelatedPerson is unable to provide care

Patient CareTeam member is a caregiver (provides ADLs). Caregiver has back strain and is unable to provide ADL support.

Note:

1. Obseration.subject is provided for relationship to the patient record.

2. When Observation.focus is populated and not a subject, should the resource documentation specify the context of the resource referenced?

Also note that RelatedPerson.subject provides context to the person who’s care team it is.