One of the design principle of FHIR is that the core resources contain data elements that service 80% of the usecases in the healthcare space. The resources in this guide leverage this design feature, making use of the core resources without any extenions (addition of elements).
The DocumentReference resource serves as the package wrapper that will house the IHE SDC Form (either base64 encoded within the resource or will be referenced via a URI). The DocumentReference will reference generated Observations, the Patient the corresponding Practitioner and any other resources that are deemed clinically relevant to the patient.
The data that is captured in a form can be viewed as observables and therefore map over logically to the Observation resource. To capture as much context as possible, the question and answer pair are captured together in an Observation.
The Observation.code will represent the question and the Observation.valueCodeableConcept will respresent a single answer. For a multiselect, the Observation.component will represent the different answers.